48 research outputs found
A Basic Unified Context for Evaluating the Beam Forming and MIMO Options in a Wireless Link
For one isolated wireless link we take a unified look at simple beamforming
(BF) as contrasted with MIMO to see how both emerge and under which conditions
advantage goes to one or the other. Communication is from a high base array to
a user in clutter. The channel propagation model is derived from fundamentals.
The base knows the power angular spectrum, but not the channel instantiation.
Eigenstates of the field spatial autocorrelation are the preferred apodizations
(APODs) which are drivers of the natural modes for exciting lectric fields.
Preference for MIMO or BF depends on APOD spectra which are surveyed pointing
to various asymptotic effects, including the maximum BF gain. Performance is
studied under varying eigenmode power settings at 10% outage. We focus on (1,4)
driving the strongest mode for BF and (4,4) driving the 4 strongest for MIMO.
Results are obtained under representative parameter settings, e.g. an angular
spread of 8 deg, 2 GHz carrier, 0 dB SNR and an array aperture of 1.68m (4
field decorrelation lengths) with antenna elements spaced as close as lambda/2.
We find MIMO excelling for array apertures much larger than the decorrelation
length; BF does almost as well for smaller apertures
Gbps User Rates Using mmWave Relayed Backhaul with High Gain Antennas
Delivering Gbps high user rate over long distances (around 1 km) is
challenging, and the abundant spectrum available in millimeter wave band cannot
solve the challenge by its own due to the severe path loss and other
limitations. Since it is economically challenging to deploy wired backhaul
every few hundred meters, relays (e.g., wireless access points) have been
proposed to extend the coverage of a base station which has wired connection to
the core network. These relays, deployed every few hundred meters, serve the
users in their vicinity and are backhauled to the base station through wireless
connections. In this work, the wireless relayed backhaul design has been
formulated as a topology-bandwidth-power joint optimization problem, and the
influence of path loss, angular spread, array size, and RF power limitation on
the user rate has been evaluated. It has been shown that for a linear network
deployed along the street at 28 GHz, when high joint directional gain (50 dBi)
is available, 1 Gbps user rate within cell range of 1 km can be delivered using
1.5 GHz of bandwidth (using single polarization antennas). The user rates drop
precipitously when joint directional gain is reduced, or when the path loss is
much more severe. When the number of RF chains is limited, the benefit of
larger arrays will eventually be surpassed by the increased channel estimation
penalty as the effective beamforming gain saturates owing to the channel
angular spread.Comment: Fixed a typo in the caption of Figure 2 ("5 dBi" should be "8 dBi"
Fundamentals of Throughput Maximization with Random Arrivals for M2M Communications
For wireless systems in which randomly arriving devices attempt to transmit a
fixed payload to a central receiver, we develop a framework to characterize the
system throughput as a function of arrival rate and per-user data rate. The
framework considers both coordinated transmission (where devices are scheduled)
and uncoordinated transmission (where devices communicate on a random access
channel and a provision is made for retransmissions). Our main contribution is
a novel characterization of the optimal throughput for the case of
uncoordinated transmission and a strategy for achieving this throughput that
relies on overlapping transmissions and joint decoding. Simulations for a
noise-limited cellular network show that the optimal strategy provides a factor
of four improvement in throughput compared to slotted aloha. We apply our
framework to evaluate more general system-level designs that account for
overhead signaling. We demonstrate that, for small payload sizes relevant for
machine-to-machine (M2M) communications (200 bits or less), a one-stage
strategy, where identity and data are transmitted optimally over the random
access channel, can support at least twice the number of devices compared to a
conventional strategy, where identity is established over an initial
random-access stage and data transmission is scheduled
Power-Efficient System Design for Cellular-Based Machine-to-Machine Communications
The growing popularity of Machine-to-Machine (M2M) communications in cellular
networks is driving the need to optimize networks based on the characteristics
of M2M, which are significantly different from the requirements that current
networks are designed to meet. First, M2M requires large number of short
sessions as opposed to small number of long lived sessions required by the
human generated traffic. Second, M2M constitutes a number of battery operated
devices that are static in locations such as basements and tunnels, and need to
transmit at elevated powers compared to the traditional devices. Third,
replacing or recharging batteries of such devices may not be feasible. All
these differences highlight the importance of a systematic framework to study
the power and energy optimal system design in the regime of interest for M2M,
which is the main focus of this paper. For a variety of coordinated and
uncoordinated transmission strategies, we derive results for the optimal
transmit power, energy per bit, and the maximum load supported by the base
station, leading to the following design guidelines: (i) frequency division
multiple access (FDMA), including equal bandwidth allocation, is sum-power
optimal in the asymptotically low spectral efficiency regime, (ii) while FDMA
is the best practical strategy overall, uncoordinated code division multiple
access (CDMA) is almost as good when the base station is lightly loaded, (iii)
the value of optimization within FDMA is in general not significant in the
regime of interest for M2M.Comment: submitted to IEEE Transactions on Wireless Communications, Jan. 201
Universal Path Gain Laws for Common Wireless Communication Environments
Simple and accurate expressions for path gain are derived from
electromagnetic fundamentals in a wide variety of common environments,
including Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS) indoor urban
canyons, urban/rural macro, outdoor-indoor and suburban streets with
vegetation. Penetration into a scattering region, sometimes aided by guiding,
is the "universal" phenomenon shared by the diverse morphologies. Root Mean
Square (RMS) errors against extensive measurements are under 5 dB, better than
3GPP models by 1-12 dB RMS, depending on environment. In urban canyons the
models have 4.7 dB RMS error, as compared to 7.9 dB from linear fit to data and
13.9/17.2 dB from LOS/NLOS 3GPP models. The theoretical path gains depend on
distance as a power law with exponents from a small set {1.5, 2, 2.5, 4},
specific to each morphology. This provides a theoretical justification for
widely used power law empirical models. Only coarse environmental data is
needed as parameters: street width, building height, vegetation depth, wall
material and antenna heights.Comment: accepted for publication in IEEE Transactions on Antennas and
Propagatio
Map-based Millimeter-Wave Channel Models: An Overview, Hybrid Modeling, Data, and Learning
Compared to the current wireless communication systems, millimeter wave
(mm-Wave) promises a wide range of spectrum. As viable alternatives to existing
mm-Wave channel models, various map-based channel models with different
modeling methods have been widely discussed. Map-based channel models are based
on a ray-tracing algorithm and include realistic channel parameters in a given
map. Such parameters enable researchers to accurately evaluate novel
technologies in the mm-Wave range. Diverse map-based modeling methods result in
different modeling objectives, including the characteristics of channel
parameters and different complexities of the modeling procedure. This article
outlines an overview of map-based mm-Wave channel models and proposes a concept
of how they can be utilized to integrate a hardware testbed/sounder with a
software testbed/sounder. In addition, we categorize map-based channel
parameters and provide guidelines for hybrid modeling. Next, we share the
measurement data and the map-based channel parameters with the public. Lastly,
we evaluate a machine learning-based beam selection algorithm through the
shared database. We expect that the offered guidelines and the shared database
will enable researchers to readily design a map-based channel model
Matching in the Air: Optimal Analog Beamforming under Angular Spread
Gbps wireless transmission over long distance at high frequency bands has
great potential for 5G and beyond, as long as high beamforming gain could be
delivered at affordable cost to combat the severe path loss. With limited
number of RF chains, the effective beamwidth of a high gain antenna will be
"widened" by channel angular spread, resulting in gain reduction. In this
paper, we formulate the analog beamforming as a constrained optimization
problem and present closed form solution that maximizes the effective
beamforming gain. The optimal beam pattern of antenna array turns out to
"match" the channel angular spread, and the effectiveness of the theoretical
results has been verified by numerical evaluation via exhaustive search and
system level simulation using 3D channel models. Furthermore, we propose an
efficient angular spread estimation method using as few as three power
measurements and validate its accuracy by lab measurements using a
phased array at 28 GHz. The capability of estimating angular
spread and matching the beam pattern on the fly enables high effective gain
using low cost analog/hybrid beamforming implementation, and we demonstrate a
few examples where substantial gain can be achieved through array geometry
optimization.Comment: Submitted to IEEE for possible publicatio
Beamforming Learning for mmWave Communication: Theory and Experimental Validation
To establish reliable and long-range millimeter-wave (mmWave) communication,
beamforming is deemed to be a promising solution. Although beamforming can be
done in the digital and analog domains, both approaches are hindered by several
constraints when it comes to mmWave communications. For example, performing
fully digital beamforming in mmWave systems involves using many radio frequency
(RF) chains, which are expensive and consume high power. This necessitates
finding more efficient ways for using fewer RF chains while taking advantage of
the large antenna arrays. One way to overcome this challenge is to employ
(partially or fully) analog beamforming through proper configuration of
phase-shifters. Existing works on mmWave analog beam design either rely on the
knowledge of the channel state information (CSI) per antenna within the array,
require a large search time (e.g., exhaustive search) or do not guarantee a
minimum beamforming gain (e.g., codebook based beamforming). In this paper, we
propose a beam design technique that reduces the search time and does not
require CSI while guaranteeing a minimum beamforming gain. The key idea derives
from observations drawn from real-life measurements. It was observed that for a
given propagation environment (e.g., coverage area of a mmWave BS) the
azimuthal angles of dominant signals could be more probable from certain angles
than others. Thus, pre-collected measurements could used to build a beamforming
codebook that regroups the most probable beam designs. We invoke Bayesian
learning for measurements clustering. We evaluate the efficacy of the proposed
scheme in terms of building the codebook and assessing its performance through
real-life measurements. We demonstrate that the training time required by the
proposed scheme is only 5% of that of exhaustive search. This crucial gain is
obtained while achieving a minimum targeted beamforming gain
Path Loss and Directional Gain Measurements at 28 GHz for non-Line-of-Sight Coverage of Indoors with Corridors
Adequate coverage with high gain antennas is key to realizing the full
promise of the bandwidth available at mm/cm wave bands. We report extensive
indoor measurements at 28 GHz (1000 links, 9.9 million individual power
measurements, 10 offices, 2 buildings), with/without line-of-sight (LOS) using
a continuous wave channel sounder, with a 10o spinning horn, capable of
capturing a full azimuth scan every 200 ms, in up to 171 dB path loss to
characterize coverage with 90% confidence level. The environment had prominent
corridors and rooms, as opposed to open/mixed offices in latest 3GPP standards.
Guiding in corridors leads to much lower RMS azimuth spread (7 degree median in
corridor non-LOS vs. 42 degree in 3GPP) and higher penetration loss into rooms
and around corners (30-32 dB, some 12 dB more loss than 3GPP at 20 m non-LOS).
Measured path gain in non-LOS is predicted by a mode-diffusion model with 3.9
dB RMS error. Scattering degraded azimuth gain by up to 4 dB in the corridor
and 7 dB in rooms with 90% probability. Link simulations in a canonical
building indicate every corridor needs an access point to provide 1 Gbps rate
to adjoining rooms within 50 m using 400 MHz of bandwidth.Comment: Accepted for publication at IEEE Transactions on Antenna and
Propagation. The channel dynamics part included in the first version of this
arXiv paper was taken out and published separately at IEEE Antennas and
Wireless Propagation Letters with title "Measurements of Beamswitching Gains
and Fade Dynamics for 28 GHz Indoor Static Links in the Presence of
Pedestrian Traffic
Suburban Residential Building Penetration Loss at 28 GHz for Fixed Wireless Access
Fixed wireless access at mm/cm bands has been proposed for high-speed
broadband access to suburban residential customers and building penetration
loss is a key parameter. We report a measurement campaign at 28 GHz in a New
Jersey suburban residential neighborhood for three representative single-family
homes. A base antenna is mounted at 3-meter height, emulating a base station on
a lamppost, moves down the street up to 200 meters. A customer premises
equipment (CPE) device, acting as relay to provide indoor coverage throughout
the desired area, is mounted either on the exterior of a street-facing window
or 1.5 meters behind the window. The median indoor-outdoor path gain
difference, corresponding to the extra loss by moving the window-mounted CPE
indoor, is found to be 9 dB for the house with low-loss materials and
plain-glass windows, and 17 dB for the house with low-emissivity windows and
foil-backed insulation. These losses are in addition to other losses (e.g.,
vegetation blockage) in comparison to free space propagation.Comment: accepted by IEEE IEEE Wireless Communications Letter