48 research outputs found
The Outage Probability of a Finite Ad Hoc Network in Nakagami Fading
An ad hoc network with a finite spatial extent and number of nodes or mobiles
is analyzed. The mobile locations may be drawn from any spatial distribution,
and interference-avoidance protocols or protection against physical collisions
among the mobiles may be modeled by placing an exclusion zone around each
radio. The channel model accounts for the path loss, Nakagami fading, and
shadowing of each received signal. The Nakagami m-parameter can vary among the
mobiles, taking any positive value for each of the interference signals and any
positive integer value for the desired signal. The analysis is governed by a
new exact expression for the outage probability, defined to be the probability
that the signal-to-interference-and-noise ratio (SINR) drops below a threshold,
and is conditioned on the network geometry and shadowing factors, which have
dynamics over much slower timescales than the fading. By averaging over many
network and shadowing realizations, the average outage probability and
transmission capacity are computed. Using the analysis, many aspects of the
network performance are illuminated. For example, one can determine the
influence of the choice of spreading factors, the effect of the receiver
location within the finite network region, and the impact of both the fading
parameters and the attenuation power laws.Comment: to appear in IEEE Transactions on Communication
Coded DS-CDMA Systems with Iterative Channel Estimation and no Pilot Symbols
In this paper, we describe direct-sequence code-division multiple-access
(DS-CDMA) systems with quadriphase-shift keying in which channel estimation,
coherent demodulation, and decoding are iteratively performed without the use
of any training or pilot symbols. An expectation-maximization
channel-estimation algorithm for the fading amplitude, phase, and the
interference power spectral density (PSD) due to the combined interference and
thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate
codes. After initial estimates of the fading amplitude, phase, and interference
PSD are obtained from the received symbols, subsequent values of these
parameters are iteratively updated by using the soft feedback from the channel
decoder. The updated estimates are combined with the received symbols and
iteratively passed to the decoder. The elimination of pilot symbols simplifies
the system design and allows either an enhanced information throughput, an
improved bit error rate, or greater spectral efficiency. The interference-PSD
estimation enables DS-CDMA systems to significantly suppress interference.Comment: To appear, IEEE Transactions on Wireless Communication
Analysis of Multi-Cell Downlink Cooperation with a Constrained Spatial Model
Multi-cell cooperation (MCC) mitigates intercell interference and improves
throughput at the cell edge. This paper considers a cooperative downlink,
whereby cell-edge mobiles are served by multiple cooperative base stations. The
cooperating base stations transmit identical signals over paths with
non-identical path losses, and the receiving mobile performs diversity
combining. The analysis in this paper is driven by a new expression for the
conditional outage probability when signals arriving over different paths are
combined in the presence of noise and interference, where the conditioning is
with respect to the network topology and shadowing. The channel model accounts
for path loss, shadowing, and Nakagami fading, and the Nakagami fading
parameters do not need to be identical for all paths. To study performance over
a wide class of network topologies, a random spatial model is adopted, and
performance is found by statistically characterizing the rates provided on the
downlinks. To model realistic networks, the model requires a minimum separation
among base stations. Having adopted a realistic model and an accurate analysis,
the paper proceeds to determine performance under several resource-allocation
policies and provides insight regarding how the cell edge should be defined.Comment: 6 pages, 3 figures, IEEE Global Telecommun. Conf. (GLOBECOM), 2013,
to appear. arXiv admin note: text overlap with arXiv:1210.366
Performance Analysis of Fifth-Generation Cellular Uplink
Fifth-generation cellular networks are expected to exhibit at least three
primary physical-layer differences relative to fourth-generation ones:
millimeter-wave propagation, antenna-array directionality, and densification of
base stations. In this paper, the effects of these differences on the
performance of single-carrier frequency-domain multiple-access uplink systems
with frequency hopping are assessed. A new analysis, which is much more
detailed than any other in the existing literature and accommodates actual
base-station topologies, captures the primary features of uplink
communications. Distance-dependent power-law, shadowing, and fading models
based on millimeter-wave measurements are introduced. The beneficial effects of
base-station densification, highly directional sectorization, and frequency
hopping are illustrated.Comment: 6 pages, 5 figures, IEEE Military Commun. Conf. (MILCOM), 201
Multihop Routing in Ad Hoc Networks
This paper presents a dual method of closed-form analysis and lightweight
simulation that enables an evaluation of the performance of mobile ad hoc
networks that is more realistic, efficient, and accurate than those found in
existing publications. Some features accommodated by the new analysis are
shadowing, exclusion and guard zones, and distance-dependent fading. Three
routing protocols are examined: least-delay, nearest-neighbor, and
maximum-progress routing. The tradeoffs among the path reliabilities, average
conditional delays, average conditional number of hops, and area spectral
efficiencies are examined.Comment: 6 pages, 6 figures, to appear in IEEE Military Commun. Conf.
(MILCOM), 201