167 research outputs found
Massive MIMO with Dual-Polarized Antennas
This paper considers a single-cell massive MIMO (multiple-input
multiple-output) system with dual-polarized antennas at both the base station
and users. We study a channel model that includes the key practical aspects
that arise when utilizing dual-polarization: channel cross-polar discrimination
(XPD) and cross-polar correlations (XPC) at the transmitter and receiver. We
derive the achievable uplink and downlink spectral efficiencies (SE) with and
without successive interference cancellation (SIC) when using the linear
minimum mean squared error (MMSE), zero-forcing (ZF), and maximum ratio (MR)
combining/precoding schemes. The expressions depend on the statistical
properties of the MMSE channel estimator obtained for the dual-polarized
channel model. Closed-form uplink and downlink SE expressions for MR
combining/precoding are derived. Using these expressions, we propose
power-control algorithms that maximize the uplink and downlink sum SEs under
uncorrelated fading but can be used to enhance performance also with correlated
fading. We compare the SEs achieved in dual-polarized and uni-polarized setups
numerically and evaluate the impact of XPD and XPC conditions. The simulations
reveal that dual-polarized setups achieve 40-60\% higher SEs and the gains
remain also under severe XPD and XPC. Dual-polarized also systems benefit more
from advanced signal processing that compensates for imperfections.Comment: 15 pages, 9 figures. To appear in IEEE Transactions on Wireless
Communication
Tri-Polarized Holographic MIMO Surface in Near-Field: Channel Modeling and Precoding Design
This paper investigates the utilization of triple polarization (TP) for
multi-user (MU) holographic multiple-input multi-output surface (HMIMOS)
wireless communication systems, targeting capacity boosting and diversity
exploitation without enlarging the antenna array sizes. We specifically
consider that both the transmitter and receiver are both equipped with an
HMIMOS consisting of compact sub-wavelength TP patch antennas within the
near-field (NF) regime. To characterize TP MU-HMIMOS systems, a TP NF channel
model is constructed using the dyadic Green's function, whose characteristics
are leveraged to design two precoding schemes for mitigating the
cross-polarization and inter-user interference contributions. Specifically, a
user-cluster-based precoding scheme assigns different users to one of three
polarizations at the expense of the system's diversity, and a two-layer
precoding scheme removes interference using the Gaussian elimination method at
a high computational cost. The theoretical correlation analysis for HMIMOS in
the NF region is also investigated, revealing that both the spacing of transmit
patch antennas and user distance impact transmit correlation factors. Our
numerical results show that the users far from transmitting HMIMOS experience
higher correlation than those closer within the NF regime, resulting in a lower
channel capacity. Meanwhile, in terms of channel capacity, TP HMIMOS can almost
achieve 1.25 times gain compared with dual-polarized HMIMOS, and 3 times
compared with conventional HMIMOS. In addition, the proposed two-layer
precoding scheme combined with two-layer power allocation realizes a higher
spectral efficiency than other schemes without sacrificing diversity
Near-Field Communications: A Comprehensive Survey
Multiple-antenna technologies are evolving towards large-scale aperture
sizes, extremely high frequencies, and innovative antenna types. This evolution
is giving rise to the emergence of near-field communications (NFC) in future
wireless systems. Considerable attention has been directed towards this
cutting-edge technology due to its potential to enhance the capacity of
wireless networks by introducing increased spatial degrees of freedom (DoFs) in
the range domain. Within this context, a comprehensive review of the state of
the art on NFC is presented, with a specific focus on its 1) fundamental
operating principles, 2) channel modeling, 3) performance analysis, 4) signal
processing, and 5) integration with other emerging technologies. Specifically,
1) the basic principles of NFC are characterized from both physics and
communications perspectives, unveiling its unique properties in contrast to
far-field communications. 2) Based on these principles, deterministic and
stochastic near-field channel models are investigated for spatially-discrete
(SPD) and continuous-aperture (CAP) antenna arrays. 3) Rooted in these models,
existing contributions on near-field performance analysis are reviewed in terms
of DoFs/effective DoFs (EDoFs), power scaling law, and transmission rate. 4)
Existing signal processing techniques for NFC are systematically surveyed,
encompassing channel estimation, beamforming design, and low-complexity beam
training. 5) Major issues and research opportunities associated with the
integration of NFC and other emerging technologies are identified to facilitate
NFC applications in next-generation networks. Promising directions are
highlighted throughout the paper to inspire future research endeavors in the
realm of NFC.Comment: 56 pages, 23figures; submit for possible journa
Holographic MIMO Communications: Theoretical Foundations, Enabling Technologies, and Future Directions
Future wireless systems are envisioned to create an endogenously
holography-capable, intelligent, and programmable radio propagation
environment, that will offer unprecedented capabilities for high spectral and
energy efficiency, low latency, and massive connectivity. A potential and
promising technology for supporting the expected extreme requirements of the
sixth-generation (6G) communication systems is the concept of the holographic
multiple-input multiple-output (HMIMO), which will actualize holographic radios
with reasonable power consumption and fabrication cost. The HMIMO is
facilitated by ultra-thin, extremely large, and nearly continuous surfaces that
incorporate reconfigurable and sub-wavelength-spaced antennas and/or
metamaterials. Such surfaces comprising dense electromagnetic (EM) excited
elements are capable of recording and manipulating impinging fields with utmost
flexibility and precision, as well as with reduced cost and power consumption,
thereby shaping arbitrary-intended EM waves with high energy efficiency. The
powerful EM processing capability of HMIMO opens up the possibility of wireless
communications of holographic imaging level, paving the way for signal
processing techniques realized in the EM-domain, possibly in conjunction with
their digital-domain counterparts. However, in spite of the significant
potential, the studies on HMIMO communications are still at an initial stage,
its fundamental limits remain to be unveiled, and a certain number of critical
technical challenges need to be addressed. In this survey, we present a
comprehensive overview of the latest advances in the HMIMO communications
paradigm, with a special focus on their physical aspects, their theoretical
foundations, as well as the enabling technologies for HMIMO systems. We also
compare the HMIMO with existing multi-antenna technologies, especially the
massive MIMO, present various...Comment: double column, 58 page
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