30 research outputs found
Open vs Closed Access Femtocells in the Uplink
Femtocells are assuming an increasingly important role in the coverage and
capacity of cellular networks. In contrast to existing cellular systems,
femtocells are end-user deployed and controlled, randomly located, and rely on
third party backhaul (e.g. DSL or cable modem). Femtocells can be configured to
be either open access or closed access. Open access allows an arbitrary nearby
cellular user to use the femtocell, whereas closed access restricts the use of
the femtocell to users explicitly approved by the owner. Seemingly, the network
operator would prefer an open access deployment since this provides an
inexpensive way to expand their network capabilities, whereas the femtocell
owner would prefer closed access, in order to keep the femtocell's capacity and
backhaul to himself. We show mathematically and through simulations that the
reality is more complicated for both parties, and that the best approach
depends heavily on whether the multiple access scheme is orthogonal (TDMA or
OFDMA, per subband) or non-orthogonal (CDMA). In a TDMA/OFDMA network,
closed-access is typically preferable at high user densities, whereas in CDMA,
open access can provide gains of more than 200% for the home user by reducing
the near-far problem experienced by the femtocell. The results of this paper
suggest that the interests of the femtocell owner and the network operator are
more compatible than typically believed, and that CDMA femtocells should be
configured for open access whereas OFDMA or TDMA femtocells should adapt to the
cellular user density.Comment: 21 pages, 8 figures, 2 tables, submitted to IEEE Trans. on Wireless
Communication
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Interference management and decentralized channel access schemes in hotspot-aided cellular networks
A system and method are provided wherein one or more femtocell base stations are deployed within a range of a cellular base station and utilize substantially the same frequency band as the cellular base station. Each femtocell base station may be configured to employ one or more interference avoidance techniques such that coexistence between the cellular and the corresponding femtocell base station is enabled. The interference avoidance techniques employed may include use of randomized time or frequency hopping; randomly selecting a predetermined number, or identifying one or more unutilized, frequency subchannels for signal transmission; using two or more transmit and two or more receive antennas; nulling one or more transmissions in a direction of a nearby cellular base station user; handing off at least one cellular user to one of the femtocell base stations and vice versa; and/or reducing the transmission power of at least one femtocell base station.Board of Regents, University of Texas Syste
Optimal decentralized spectral resource allocation for OFDMA downlink of femto networks via adaptive gradient vector step size approach
For the orthogonal frequency division multiple access (OFDMA) downlink of a femto network, the resource allocation scheme would aim to maximize the area spectral efficiency (ASE) subject to constraints on the radio resources per transmission interval accessible by each femtocell. An optimal resource allocation scheme for completely decentralized femtocell deployments leads to a nonlinear optimization problem because the cost function of the optimization problem is nonlinear. In this paper, an adaptive gradient vector step size approach is proposed for finding the optimal solution of the optimization problem. Computer numerical simulation results show that our proposed method is more efficient than existing exhaustive search methods
Femtocell Networks: A Survey
The surest way to increase the system capacity of a wireless link is by
getting the transmitter and receiver closer to each other, which creates the
dual benefits of higher quality links and more spatial reuse. In a network with
nomadic users, this inevitably involves deploying more infrastructure,
typically in the form of microcells, hotspots, distributed antennas, or relays.
A less expensive alternative is the recent concept of femtocells, also called
home base-stations, which are data access points installed by home users get
better indoor voice and data coverage. In this article, we overview the
technical and business arguments for femtocells, and describe the
state-of-the-art on each front. We also describe the technical challenges
facing femtocell networks, and give some preliminary ideas for how to overcome
them.Comment: IEEE Communications Magazine, vol. 46, no.9, pp. 59-67, Sept. 200
Coverage in Multi-Antenna Two-Tier Networks
In two-tier networks -- comprising a conventional cellular network overlaid
with shorter range hotspots (e.g. femtocells, distributed antennas, or wired
relays) -- with universal frequency reuse, the near-far effect from cross-tier
interference creates dead spots where reliable coverage cannot be guaranteed to
users in either tier. Equipping the macrocell and femtocells with multiple
antennas enhances robustness against the near-far problem. This work derives
the maximum number of simultaneously transmitting multiple antenna femtocells
meeting a per-tier outage probability constraint. Coverage dead zones are
presented wherein cross-tier interference bottlenecks cellular and hotspot
coverage. Two operating regimes are shown namely 1) a cellular-limited regime
in which femtocell users experience unacceptable cross-tier interference and 2)
a hotspot-limited regime wherein both femtocell users and cellular users are
limited by hotspot interference. Our analysis accounts for the per-tier
transmit powers, the number of transmit antennas (single antenna transmission
being a special case) and terrestrial propagation such as the Rayleigh fading
and the path loss exponents. Single-user (SU) multiple antenna transmission at
each tier is shown to provide significantly superior coverage and spatial reuse
relative to multiuser (MU) transmission. We propose a decentralized
carrier-sensing approach to regulate femtocell transmission powers based on
their location. Considering a worst-case cell-edge location, simulations using
typical path loss scenarios show that our interference management strategy
provides reliable cellular coverage with about 60 femtocells per cellsite.Comment: 30 Pages, 11 figures, Revised and Resubmitted to IEEE Transactions on
Wireless Communication
Power Control in Two-Tier Femtocell Networks
In a two tier cellular network -- comprised of a central macrocell underlaid
with shorter range femtocell hotspots -- cross-tier interference limits overall
capacity with universal frequency reuse. To quantify near-far effects with
universal frequency reuse, this paper derives a fundamental relation providing
the largest feasible cellular Signal-to-Interference-Plus-Noise Ratio (SINR),
given any set of feasible femtocell SINRs. We provide a link budget analysis
which enables simple and accurate performance insights in a two-tier network. A
distributed utility-based SINR adaptation at femtocells is proposed in order to
alleviate cross-tier interference at the macrocell from cochannel femtocells.
The Foschini-Miljanic (FM) algorithm is a special case of the adaptation. Each
femtocell maximizes their individual utility consisting of a SINR based reward
less an incurred cost (interference to the macrocell). Numerical results show
greater than 30% improvement in mean femtocell SINRs relative to FM. In the
event that cross-tier interference prevents a cellular user from obtaining its
SINR target, an algorithm is proposed that reduces transmission powers of the
strongest femtocell interferers. The algorithm ensures that a cellular user
achieves its SINR target even with 100 femtocells/cell-site, and requires a
worst case SINR reduction of only 16% at femtocells. These results motivate
design of power control schemes requiring minimal network overhead in two-tier
networks with shared spectrum.Comment: 29 pages, 10 figures, Revised and resubmitted to the IEEE
Transactions on Wireless Communication
A network approach to public goods
Abstract We study settings where each agent can exert costly effort that creates nonrival, heterogeneous benefits for some of the others. For example, municipalities can forgo consumption to reduce pollution. How do the prospects for efficient cooperation depend on asymmetries in the effects of players' actions? We approach this question by analyzing a network that describes the marginal benefits agents can confer on one another. The first set of results explains how the largest eigenvalue of this network measures the marginal gains available from cooperating; as an application, we describe the players whose participation is essential to achieving any Pareto improvement on an inefficient status quo. Next, we examine mechanisms all of whose equilibria are Pareto efficient and individually rational; an outcome is called robust if it is an equilibrium outcome in every such mechanism. Robust outcomes exist and correspond to the Lindahl public goods solutions. The main result is a characterization of effort levels at these outcomes in terms of players' centralities in the benefits network. It entails that an outcome is robust if and only if agents contribute in proportion to how much they value the efforts of those who help them
Coexistence in femtocell-aided cellular architectures
textThe surest way to increase the capacity of a wireless system is by getting the
transmitters and receivers closer to each other, which creates the dual bene¯ts of
higher quality links and more spatial reuse. In a network with nomadic users, this
inevitably involves deploying more infrastructure, typically in the form of microcells,
hotspots, distributed antennas, or relays. Compared to these deployments, a less
expensive alternative for cellular operators is the recent concept of femtocells { also
called home base-stations { which are end consumer installed data access points in
the desire to get better indoor voice and data coverage. A two-tier network consisting
of a conventional macrocell overlaid with shorter range wireless hotspots o®ers poten-
tial capacity bene¯ts with low upfront costs to cellular operators. This dissertation
addresses the key technical challenges inherent to a femtocell-aided cellular network,
speci¯cally managing radio interference and providing reliable coverage at either tier,
for di®erent physical layer technologies. Speci¯c contributions include 1) an uplink
capacity analysis and interference avoidance in two-tier networks employing Code Di-
vision Multiple Access (CDMA), 2) a decentralized power control scheme in two-tier
networks with universal frequency reuse, 3) a coverage analysis of multi-antenna two-
tier networks, and 4) spectrum allocation in two-tier networks employing Orthogonal
Frequency Division Multiple Access (OFDMA). The goal of this research is to inspire and motivate the use of decentralized interference management techniques requir-
ing minimal network overhead in ongoing and future deployments of tiered cellular
architectures.Electrical and Computer Engineerin