3,735 research outputs found
Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance
In massive MIMO, most precoders result in downlink signals that suffer from
high PAR, independently of modulation order and whether single-carrier or OFDM
transmission is used. The high PAR lowers the power efficiency of the base
station amplifiers. To increase power efficiency, low-PAR precoders have been
proposed. In this article, we compare different transmission schemes for
massive MIMO in terms of the power consumed by the amplifiers. It is found that
(i) OFDM and single-carrier transmission have the same performance over a
hardened massive MIMO channel and (ii) when the higher amplifier power
efficiency of low-PAR precoding is taken into account, conventional and low-PAR
precoders lead to approximately the same power consumption. Since downlink
signals with low PAR allow for simpler and cheaper hardware, than signals with
high PAR, therefore, the results suggest that low-PAR precoding with either
single-carrier or OFDM transmission should be used in a massive MIMO base
station
Massive MU-MIMO Downlink TDD Systems with Linear Precoding and Downlink Pilots
We consider a massive MU-MIMO downlink time-division duplex system where a
base station (BS) equipped with many antennas serves several single-antenna
users in the same time-frequency resource. We assume that the BS uses linear
precoding for the transmission. To reliably decode the signals transmitted from
the BS, each user should have an estimate of its channel. In this work, we
consider an efficient channel estimation scheme to acquire CSI at each user,
called beamforming training scheme. With the beamforming training scheme, the
BS precodes the pilot sequences and forwards to all users. Then, based on the
received pilots, each user uses minimum mean-square error channel estimation to
estimate the effective channel gains. The channel estimation overhead of this
scheme does not depend on the number of BS antennas, and is only proportional
to the number of users. We then derive a lower bound on the capacity for
maximum-ratio transmission and zero-forcing precoding techniques which enables
us to evaluate the spectral efficiency taking into account the spectral
efficiency loss associated with the transmission of the downlink pilots.
Comparing with previous work where each user uses only the statistical channel
properties to decode the transmitted signals, we see that the proposed
beamforming training scheme is preferable for moderate and low-mobility
environments.Comment: Allerton Conference on Communication, Control, and Computing,
Urbana-Champaign, Illinois, Oct. 201
Out-of-Band Radiation Measure for MIMO Arrays with Beamformed Transmission
The spatial characteristics of the out-of-band radiation that a multiuser
MIMO system emits in the environment, due to its power amplifiers (modeled by a
polynomial model) are nonlinear, is studied by deriving an analytical
expression for the continuous-time cross-correlation of the transmit signals.
At a random spatial point, the same power is received at any frequency on
average with a MIMO base station as with a SISO base station when the two
radiate the same amount of power. For a specific channel realization however,
the received power depends on the channel. We show that the power received
out-of-band only deviates little from the average in a MIMO system with
multiple users and that the deviation can be significant with only one user.
Using an ergodicity argument, we conclude that out-of-band radiation is less of
a problem in massive MIMO, where total radiated power is lower compared to SISO
systems and that requirements on spectral regrowth can be relaxed in MIMO
systems without causing more total out-of-band radiation
Impact of Spatial Filtering on Distortion from Low-Noise Amplifiers in Massive MIMO Base Stations
In massive MIMO base stations, power consumption and cost of the low-noise
amplifiers (LNAs) can be substantial because of the many antennas. We
investigate the feasibility of inexpensive, power efficient LNAs, which
inherently are less linear. A polynomial model is used to characterize the
nonlinear LNAs and to derive the second-order statistics and spatial
correlation of the distortion. We show that, with spatial matched filtering
(maximum-ratio combining) at the receiver, some distortion terms combine
coherently, and that the SINR of the symbol estimates therefore is limited by
the linearity of the LNAs. Furthermore, it is studied how the power from a
blocker in the adjacent frequency band leaks into the main band and creates
distortion. The distortion term that scales cubically with the power received
from the blocker has a spatial correlation that can be filtered out by spatial
processing and only the coherent term that scales quadratically with the power
remains. When the blocker is in free-space line-of-sight and the LNAs are
identical, this quadratic term has the same spatial direction as the desired
signal, and hence cannot be removed by linear receiver processing
Spatial Characteristics of Distortion Radiated from Antenna Arrays with Transceiver Nonlinearities
The distortion from massive MIMO (multiple-input--multiple-output) base
stations with nonlinear amplifiers is studied and its radiation pattern is
derived. The distortion is analyzed both in-band and out-of-band. By using an
orthogonal Hermite representation of the amplified signal, the spatial
cross-correlation matrix of the nonlinear distortion is obtained. It shows
that, if the input signal to the amplifiers has a dominant beam, the distortion
is beamformed in the same way as that beam. When there are multiple beams
without any one being dominant, it is shown that the distortion is practically
isotropic. The derived theory is useful to predict how the nonlinear distortion
will behave, to analyze the out-of-band radiation, to do reciprocity
calibration, and to schedule users in the frequency plane to minimize the
effect of in-band distortion
Aspects of Favorable Propagation in Massive MIMO
Favorable propagation, defined as mutual orthogonality among the
vector-valued channels to the terminals, is one of the key properties of the
radio channel that is exploited in Massive MIMO. However, there has been little
work that studies this topic in detail. In this paper, we first show that
favorable propagation offers the most desirable scenario in terms of maximizing
the sum-capacity. One useful proxy for whether propagation is favorable or not
is the channel condition number. However, this proxy is not good for the case
where the norms of the channel vectors may not be equal. For this case, to
evaluate how favorable the propagation offered by the channel is, we propose a
``distance from favorable propagation'' measure, which is the gap between the
sum-capacity and the maximum capacity obtained under favorable propagation.
Secondly, we examine how favorable the channels can be for two extreme
scenarios: i.i.d. Rayleigh fading and uniform random line-of-sight (UR-LoS).
Both environments offer (nearly) favorable propagation. Furthermore, to analyze
the UR-LoS model, we propose an urns-and-balls model. This model is simple and
explains the singular value spread characteristic of the UR-LoS model well
Massive MIMO for Next Generation Wireless Systems
Multi-user Multiple-Input Multiple-Output (MIMO) offers big advantages over
conventional point-to-point MIMO: it works with cheap single-antenna terminals,
a rich scattering environment is not required, and resource allocation is
simplified because every active terminal utilizes all of the time-frequency
bins. However, multi-user MIMO, as originally envisioned with roughly equal
numbers of service-antennas and terminals and frequency division duplex
operation, is not a scalable technology. Massive MIMO (also known as
"Large-Scale Antenna Systems", "Very Large MIMO", "Hyper MIMO", "Full-Dimension
MIMO" & "ARGOS") makes a clean break with current practice through the use of a
large excess of service-antennas over active terminals and time division duplex
operation. Extra antennas help by focusing energy into ever-smaller regions of
space to bring huge improvements in throughput and radiated energy efficiency.
Other benefits of massive MIMO include the extensive use of inexpensive
low-power components, reduced latency, simplification of the media access
control (MAC) layer, and robustness to intentional jamming. The anticipated
throughput depend on the propagation environment providing asymptotically
orthogonal channels to the terminals, but so far experiments have not disclosed
any limitations in this regard. While massive MIMO renders many traditional
research problems irrelevant, it uncovers entirely new problems that urgently
need attention: the challenge of making many low-cost low-precision components
that work effectively together, acquisition and synchronization for
newly-joined terminals, the exploitation of extra degrees of freedom provided
by the excess of service-antennas, reducing internal power consumption to
achieve total energy efficiency reductions, and finding new deployment
scenarios. This paper presents an overview of the massive MIMO concept and
contemporary research.Comment: Final manuscript, to appear in IEEE Communications Magazin
The application of fuzzy text recognition and -manipulation technologies to clean-up, idealize, improve, and integrate sets of unstructured data
Master's thesis in Computer scienceThe application of fuzzy text recognition and -manipulation technologies to clean-up, idealize, improve, and integrate sets of unstructured data
Nilpferd und Papyrusdickicht in den Gräbern des Alten Reiches
Competence Portals are software tools that are intended to make it easier for persons that have had no previous contact to find and contact each other. The portals can address areas ranging from finding an expert within an organisation to the marketing of the competence of a region or a research centre to other organisations. The purpose of the thesis is to examine the necessity of Competence Portals in research-intensive organisations. Important characteristics of the information in such portals have been identified and used as a basis for recommendations regarding how Competence Portals could be designed. Furthermore, the thesis focuses on the task of finding a source of knowledge within an organisation. The study is of a general nature and is supposed to be of interest to anyone who has an interest in knowledge management and tools to enable easy contacts within and between organisations. The study includes research organisations from Sweden, Germany and the United Kingdom and privately owned Swedish companies. The focus is on the German research organisations and Swedish companies. The empirical data was gathered using two surveys and multiple interviews with both persons featured in a Competence Portal as well as prospective users. We have found that is uncommon to have access to tools such as Competence Portals in the studied groups. The tasks that a portal is meant to make easier is most often already solved in an efficient way or not performed frequently enough to merit a larger investment. Furthermore, the general opinion regarding Competence Systems in the studied group is very sceptical. We have therefore reached the conclusion that the demand for a software solution such as Competence Portals is low in the studied group. As a result of the little room for improvement in current work procedures and a general low demand we conclude that the necessity of Competence Portals is low in the studied group. However, Competence Portals are likely to be useful to very large or geographically scattered organisations. We have summarized our findings regarding the quality aspects of the information inCompetence Portals in a model that illustrates the important areas to consider when designing a Competence Portal. The model emphasise the importance of processes for updating and maintaining the information in the portal
- …