Heedong Do

A relentless trend in wireless communications is the hunger for bandwidth, and fresh bandwidth is only to be found at ever higher frequencies. While 5G systems are seizing the mmWave band, the attention of researchers is shifting already to the terahertz range. In that distant land of tiny wavelengths, antenna arrays can serve for more than power-enhancing beamforming. Defying lower-frequency wisdom, spatial multiplexing becomes feasible even in line-of-sight conditions. This article reviews the

This paper establishes an upper bound on the capacity of line-of-sight multiantenna channels over all possible antenna arrangements and shows that uniform linear arrays (ULAs) with an SNR-dependent rotation of transmitter and/or receiver can closely approach such capacity-and in fact achieve it at low and high SNR, and asymptotically in the numbers of antennas. Then, as an alternative to mechanically rotating ULAs, we propose to electronically select among multiple ULAs having a radial dispositi

This paper deals with line-of-sight (LOS) MIMO communication via an intelligent reflecting surface (IRS). It is shown that the number of spatial degrees of freedom (DOF) afforded by this setting grows in proportion with the IRS aperture, as opposed to being dictated by the transmit and receive apertures; this buttresses the interest in IRS deployments at mmWave and terahertz frequencies, with wavelengths and transmission ranges small enough to enable LOS MIMO. Explicit and simple-to-implement IR

Motivated by the widespread adoption of the parabolic wavefront model for line-of-sight (LOS) multiple-input multiple-output (MIMO) communication, this paper presents a comprehensive analysis of this model’s validity and simple conditions that ensure its applicability. Then, with the model’s scope clearly delineated, the paper expounds a number of properties of the channel that results from applying it. Connections are drawn among these properties under the umbrella of a Fourier interpretation,

We establish an upper bound on the information-theoretic capacity of line-of-sight (LOS) multiantenna channels with arbitrary antenna arrangements and identify array structures that, properly configured, can attain at least 96.6% of such capacity at every signal-to-noise ratio (SNR). In the process, we determine how to configure the arrays as a function of the SNR. At low- and high-SNR specifically, the configured arrays revert to simpler structures and become capacity-achieving.

With increasing frequencies, bandwidths, and array apertures, the phenomenon of beam squint arises as a serious impairment to beamforming. Fully digital arrays with true time delay per antenna element are a potential solution, but they require downconversion at each element. This paper shows that hybrid arrays can perform essentially as well as digital arrays once the number of radio-frequency chains exceeds a certain threshold that is far below the number of elements. This threshold is determin

This paper considers line-of-sight multiple-input multiple-output communication, of interest at millimeter-wave and sub-terahertz frequencies. Building on how, with an angular rotation dependent on the signal-to-noise ratio (SNR), uniform linear arrays (ULAs) can tightly approach the capacity of such channels, we assess the performance of rotatable uniform rectangular arrays (URAs). The changeover from ULAs to URAs is motivated by the interest in reducing the array footprints. For both isotropic

A relentless trend in wireless communications is the hunger for bandwidth, and fresh bandwidth is only to be found at ever-higher frequencies. While 5G systems are seizing the mmWave band, the attention of researchers is shifting already to the terahertz range. In that distant land of tiny wavelengths, antenna arrays can serve for more than power-enhancing beamforming. Defying lower-frequency wisdom, spatial multiplexing becomes feasible even in line-of-sight conditions. This paper reviews the u

This paper establishes an upper bound on the capacity of line-of-sight multiantenna channels over all possible antenna arrangements and shows that uniform linear arrays (ULAs) with an SNR-dependent rotation of transmitter or receiver can closely approach such capacity---and in fact achieve it at low and high SNR, and asymptotically in the numbers of antennas. Then, as an alternative to mechanically rotating ULAs, we propose to electronically select among multiple ULAs having a radial disposition

This paper analyzes the effect of deploying an intelligent reflective surface (IRS) in line-of-sight conditions at sub-terahertz frequencies. Besides the more obvious benefits of enhancing the received power and sidestepping blockages, at these frequencies an IRS can augment the number of spatial degrees of freedom even as the transmitter and receiver footprints remain fixed. This possibility, which is revealed only if the curved nature of the wavefronts is accounted for, results from the IRS ac

This paper studies the optimization of an active reconfigurable intelligent surface (RIS) under two power budget models. Two algorithms, one for each power constraint, for finding the optimal RIS configuration are proposed under the proviso that both the transmitter and receiver are equipped with a single antenna each. The computational complexities of these methods are linear in the number of RIS elements, and the resultant configurations are globally optimal.

As the frequency and array apertures grow large, and the transmission range shrinks, the far-field assumption ceases to hold and, with that, the wavefront curvature is revealed over the arrays. This paper deals with the estimation of multiantenna channels in the line-of-sight conditions that are prevalent in the near field. By expressing the curved wavefronts as a polynomial via a power series expansion of a sphere, the estimation of the channel over the array can be formulated as a multidimensi

Motivated by the widespread adoption of the parabolic wavefront model for the analysis and characterization of near-field multiple-input multiple-output (MIMO) communication and positioning, this paper presents an extensive analysis of this model's validity. Insightful relationships are derived for the combinations of transmission range, wavelength, and array apertures that ensure that the model is valid to within some desired accuracy. Furthermore, a connection is drawn with the number of spati

An estimation method is presented for polynomial phase signals, i.e., those adopting the form of a complex exponential whose phase is polynomial in its indices. Transcending the scope of existing techniques, the proposed estimator can handle an arbitrary number of dimensions and an arbitrary set of polynomial degrees along each dimension; the only requirement is that the number of observations per dimension exceeds the highest degree thereon. Embodied by a highly compact sequential algorithm, th

This research sets forth a universal framework to characterize the beamforming gain achievable with arbitrarily nonideal phase shifters. Precisely, the maximum possible shortfall relative to the gain attainable with ideal phase shifters is established. Such shortfall is shown to be fundamentally determined by the perimeter of the convex hull of the set of feasible beamforming coefficients on the complex plane. This result holds regardless of whether the beamforming is at the transmitter, at the