181 research outputs found
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
Efficient DSP and Circuit Architectures for Massive MIMO: State-of-the-Art and Future Directions
Massive MIMO is a compelling wireless access concept that relies on the use
of an excess number of base-station antennas, relative to the number of active
terminals. This technology is a main component of 5G New Radio (NR) and
addresses all important requirements of future wireless standards: a great
capacity increase, the support of many simultaneous users, and improvement in
energy efficiency. Massive MIMO requires the simultaneous processing of signals
from many antenna chains, and computational operations on large matrices. The
complexity of the digital processing has been viewed as a fundamental obstacle
to the feasibility of Massive MIMO in the past. Recent advances on
system-algorithm-hardware co-design have led to extremely energy-efficient
implementations. These exploit opportunities in deeply-scaled silicon
technologies and perform partly distributed processing to cope with the
bottlenecks encountered in the interconnection of many signals. For example,
prototype ASIC implementations have demonstrated zero-forcing precoding in real
time at a 55 mW power consumption (20 MHz bandwidth, 128 antennas, multiplexing
of 8 terminals). Coarse and even error-prone digital processing in the antenna
paths permits a reduction of consumption with a factor of 2 to 5. This article
summarizes the fundamental technical contributions to efficient digital signal
processing for Massive MIMO. The opportunities and constraints on operating on
low-complexity RF and analog hardware chains are clarified. It illustrates how
terminals can benefit from improved energy efficiency. The status of technology
and real-life prototypes discussed. Open challenges and directions for future
research are suggested.Comment: submitted to IEEE transactions on signal processin
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
Can Hardware Distortion Correlation be Neglected When Analyzing Uplink SE in Massive MIMO?
This paper analyzes how the distortion created by hardware impairments in a
multiple-antenna base station affects the uplink spectral efficiency (SE), with
focus on Massive MIMO. The distortion is correlated across the antennas, but
has been often approximated as uncorrelated to facilitate (tractable) SE
analysis. To determine when this approximation is accurate, basic properties of
the distortion correlation are first uncovered. Then, we focus on third-order
non-linearities and prove analytically and numerically that the correlation can
be neglected in the SE analysis when there are many users. In i.i.d. Rayleigh
fading with equal signal-to-noise ratios, this occurs when having five users.Comment: 5 pages, 3 figures, IEEE International Workshop on Signal Processing
Advances in Wireless Communications (SPAWC), 201
Out-of-Band Radiation from Antenna Arrays Clarified
Non-linearities in radio-frequency (RF) transceiver hardware, particularly in
power amplifiers, cause distortion in-band and out-of-band. Contrary to claims
made in recent literature, in a multiple-antenna system this distortion is
correlated across the antennas in the array. A significant implication of this
fact is that out-of-band emissions caused by non-linearities are beamformed, in
some cases into the same direction as the useful signal.Comment: IEEE Wireless Communications Letters, 2018, to appea
Caracterização não-linear de agregados de antenas para aplicações 5G
The present mobile scenario demands are stretching the existing telecom infrastructure to the limit. New technologies centred around antenna arrays and spatial multiplexing have been proposed to overcome the challenges imposed by these demands. This work overviews the mobile scenario, scrutinizing demands, presented solutions, challenges and the industry’s perspective of the Fifth Generation of mobile communications. From a careful analysis, the 5G’s most critical radio frequency hardware issues are detailed, and a long-term approach to address them is presented. On the short-term the work focuses on antenna characterization, because antennas are a central part of future wireless communications. Initially, basic antenna concepts are presented, then emphasis is given to microstrip antennas, going through all the steps of designing, optimizing and measuring a rectangular microstrip antenna and an eight element linear antenna array for 5.67GHz. Array features such as scanning and source synthesis are also explored. Finally, the impact of signal nonlinear distortion on the antenna array pattern is studied, aiming to expand state-of-the-art knowledge on how signal nonlinear distortion can limit spatial multiplexing. A theoretical model of the phenomenon is proposed and validated both by electromagnetic simulation and measurements.As crescentes exigências das redes móveis estão a levar a infraestrutura de telecomunicações ao seu limite. Novas tecnologias centradas em agregados de antenas e multiplexagem espacial têm sido propostas para ultrapassar os desafios impostos por tais exigências. Este trabalho apresenta uma visão abrangente das redes móveis atuais, escrutinando as suas exigências, as soluções apresentadas, os desafios adjacentes, bem como a opinião da indústria. Os problemas mais crı́ticos do hardware de radio frequência para a quinta geração de redes móveis são apurados a partir de uma análise detalhada do cenário das redes sem fios, sendo apresentado um plano a longo prazo para abordar estas problemáticas. A curto prazo o trabalho foca-se em caracterização de antenas, visto que as antenas são um ponto central nas comunicações sem fios do futuro. Inicialmente são apresentados conceitos básicos sobre antenas, dando-se de seguida ênfase às antenas microstrip, sendo apresentado todo o processo de sı́ntese, otimização e caracterização de uma antena microstrip retangular e de um agregado de antenas linear de oito elementos com frequência de operação 5.67GHz. Neste âmbito, algumas propriedades dos agregados, como o varrimento angular do feixe eletromagnético e técnicas de sı́ntese de fonte eletromagnética, são também exploradas. Finalmente, apresenta-se um estudo sobre o impacto que a distorção não linear de sinal pode ter no diagrama de radiação do agregado de antenas. O objetivo é expandir os conhecimentos do estado-da-arte acerca das limitações que a distorção não linear pode impor na multiplexagem espacial. Neste sentido, um modelo teórico descritivo deste fenómeno é proposto e validado por simulação eletromagnética e por medições experimentais.Mestrado em Engenharia Eletrónica e Telecomunicaçõe
Distortion-Aware Linear Precoding for Massive MIMO Downlink Systems with Nonlinear Power Amplifiers
We introduce a framework for linear precoder design over a massive
multiple-input multiple-output downlink system in the presence of nonlinear
power amplifiers (PAs). By studying the spatial characteristics of the
distortion, we demonstrate that conventional linear precoding techniques steer
nonlinear distortions towards the users. We show that, by taking into account
PA nonlinearity, one can design linear precoders that reduce, and in
single-user scenarios, even completely remove the distortion transmitted in the
direction of the users. This, however, is achieved at the price of a reduced
array gain. To address this issue, we present precoder optimization algorithms
that simultaneously take into account the effects of array gain, distortion,
multiuser interference, and receiver noise. Specifically, we derive an
expression for the achievable sum rate and propose an iterative algorithm that
attempts to find the precoding matrix which maximizes this expression.
Moreover, using a model for PA power consumption, we propose an algorithm that
attempts to find the precoding matrix that minimizes the consumed power for a
given minimum achievable sum rate. Our numerical results demonstrate that the
proposed distortion-aware precoding techniques provide significant improvements
in spectral and energy efficiency compared to conventional linear precoders.Comment: 30 pages, 10 figure
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