12 research outputs found
On the Effect of Mutual Coupling in One-Bit Spatial Sigma-Delta Massive MIMO Systems
The one-bit spatial Sigma-Delta concept has recently been proposed as an
approach for achieving low distortion and low power consumption for massive
multi-input multi-output systems. The approach exploits users located in known
angular sectors or spatial oversampling to shape the quantization noise away
from desired directions of arrival. While reducing the antenna spacing
alleviates the adverse impact of quantization noise, it can potentially
deteriorate the performance of the massive array due to excessive mutual
coupling. In this paper, we analyze the impact of mutual coupling on the uplink
spectral efficiency of a spatial one-bit Sigma-Delta massive MIMO architecture,
and compare the resulting performance degradation to standard one-bit
quantization as well as the ideal case with infinite precision. Our simulations
show that the one-bit Sigma-Delta array is particularly advantageous in
space-constrained scenarios, can still provide significant gains even in the
presence of mutual coupling when the antennas are closely spaced.Comment: Presented in SPAWC 202
Transmitting Data Through Reconfigurable Intelligent Surface: A Spatial Sigma-Delta Modulation Approach
Transmitting data using the phases on reconfigurable intelligent surfaces
(RIS) is a promising solution for future energy-efficient communication
systems. Recent work showed that a virtual phased massive multiuser
multiple-input-multiple-out (MIMO) transmitter can be formed using only one
active antenna and a large passive RIS. In this paper, we are interested in
using such a system to perform MIMO downlink precoding. In this context, we may
not be able to apply conventional MIMO precoding schemes, such as the simple
zero-forcing (ZF) scheme, and we typically need to design the phase signals by
solving optimization problems with constant modulus constraints or with
discrete phase constraints, which pose challenges with high computational
complexities. In this work, we propose an alternative approach based on
Sigma-Delta () modulation, which is classically famous for its
noise-shaping ability. Specifically, first-order modulation is
applied in the spatial domain to handle phase quantization in generating
constant envelope signals. Under some mild assumptions, the proposed phased
modulator allows us to use the ZF scheme to synthesize the RIS
reflection phases with negligible complexity. The proposed approach is
empirically shown to achieve comparable bit error rate performance to the
unquantized ZF scheme
Spectral Efficiency of One-Bit Sigma-Delta Massive MIMO
We examine the uplink spectral efficiency of a massive MIMO base station employing a one-bit Sigma-Delta ( \Sigma \Delta ) sampling scheme implemented in the spatial rather than the temporal domain. Using spatial rather than temporal oversampling, and feedback of the quantization error between adjacent antennas, the method shapes the spatial spectrum of the quantization noise away from an angular sector where the signals of interest are assumed to lie. It is shown that, while a direct Bussgang analysis of the \Sigma \Delta approach is not suitable, an alternative equivalent linear model can be formulated to facilitate an analysis of the system performance. The theoretical properties of the spatial quantization noise power spectrum are derived for the \Sigma \Delta array, as well as an expression for the spectral efficiency of maximum ratio combining (MRC). Simulations verify the theoretical results and illustrate the significant performance gains offered by the \Sigma \Delta approach for both MRC and zero-forcing receivers
1-Bit Massive MIMO Transmission: Embracing Interference with Symbol-Level Precoding
The deployment of large-scale antenna arrays for cellular base stations
(BSs), termed as `Massive MIMO', has been a key enabler for meeting the
ever-increasing capacity requirement for 5G communication systems and beyond.
Despite their promising performance, fully-digital massive MIMO systems require
a vast amount of hardware components including radio frequency chains, power
amplifiers, digital-to-analog converters (DACs), etc., resulting in a huge
increase in terms of the total power consumption and hardware costs for
cellular BSs. Towards both spectrally-efficient and energy-efficient massive
MIMO deployment, a number of hardware limited architectures have been proposed,
including hybrid analog-digital structures, constant-envelope transmission, and
use of low-resolution DACs. In this paper, we overview the recent interest in
improving the error-rate performance of massive MIMO systems deployed with
1-bit DACs through precoding at the symbol level. This line of research goes
beyond traditional interference suppression or cancellation techniques by
managing interference on a symbol-by-symbol basis. This provides unique
opportunities for interference-aware precoding tailored for practical massive
MIMO systems. Firstly, we characterize constructive interference (CI) and
elaborate on how CI can benefit the 1-bit signal design by exploiting the
traditionally undesired multi-user interference as well as the interference
from imperfect hardware components. Subsequently, we overview several solutions
for 1-bit signal design to illustrate the gains achievable by exploiting CI.
Finally, we identify some challenges and future research directions for 1-bit
massive MIMO systems that are yet to be explored.Comment: This work has been submitted to the IEEE for possible publication.
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Symbol-Level Precoding Through the Lens of Zero Forcing and Vector Perturbation
Symbol-level precoding (SLP) has recently emerged as a new paradigm for
physical-layer transmit precoding in multiuser multi-input-multi-output (MIMO)
channels. It exploits the underlying symbol constellation structure, which the
conventional paradigm of linear precoding does not, to enhance symbol-level
performance such as symbol error probability (SEP). It also allows the precoder
to take a more general form than linear precoding. This paper aims to better
understand the relationships between SLP and linear precoding, subsequent
design implications, and further connections beyond the existing SLP scope.
Focused on the quadrature amplitude modulation (QAM) constellations, our study
is built on a basic signal observation, namely, that SLP can be equivalently
represented by a zero-forcing (ZF) linear precoding scheme augmented with some
appropriately chosen symbol-dependent perturbation terms, and that some
extended form of SLP is equivalent to a vector perturbation (VP) nonlinear
precoding scheme augmented with the above-noted perturbation terms. We examine
how insights arising from this perturbed ZF and VP interpretations can be
leveraged to i) substantially simplify the optimization of certain SLP design
criteria, namely, total or peak power minimization subject to SEP quality
guarantees; and ii) draw connections with some existing SLP designs. We also
touch on the analysis side by showing that, under the total power minimization
criterion, the basic ZF scheme is a near-optimal SLP scheme when the QAM order
is very high -- which gives a vital implication that SLP is more useful for
lower-order QAM cases. Numerical results further indicate the merits and
limitations of the different SLP designs derived from the perturbed ZF and VP
interpretations
A Spatial Sigma-Delta Approach to Mitigation of Power Amplifier Distortions in Massive MIMO Downlink
In massive multiple-input multiple-output (MIMO) downlink systems, the
physical implementation of the base stations (BSs) requires the use of cheap
and power-efficient power amplifiers (PAs) to avoid high hardware cost and high
power consumption. However, such PAs usually have limited linear amplification
ranges. Nonlinear distortions arising from operation beyond the linear
amplification ranges can significantly degrade system performance. Existing
approaches to handle the nonlinear distortions, such as digital predistortion
(DPD), typically require accurate knowledge, or acquisition, of the PA transfer
function. In this paper, we present a new concept for mitigation of the PA
distortions. Assuming a uniform linear array (ULA) at the BS, the idea is to
apply a Sigma-Delta () modulator to spatially shape the PA
distortions to the high-angle region. By having the system operating in the
low-angle region, the received signals are less affected by the PA distortions.
To demonstrate the potential of this spatial approach, we study
the application of our approach to the multi-user MIMO-orthogonal frequency
division modulation (OFDM) downlink scenario. A symbol-level precoding (SLP)
scheme and a zero-forcing (ZF) precoding scheme, with the new design
requirement by the spatial approach being taken into account,
are developed. Numerical simulations are performed to show the effectiveness of
the developed precoding schemes