Angel Lozano’s research while affiliated with Pompeu Fabra University and other places

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Publications (180)


Multidimensional Polynomial Phase Estimation
  • Preprint

November 2024

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4 Reads

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Angel Lozano

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, this estimator exhibits a strictly linear computational complexity in the number of observations, and is efficient at high signal-to-noise ratios (SNRs). To reinforce the performance at low and medium SNRs, where any phase estimator is bound to be hampered by the inherent ambiguity caused by phase wrappings, suitable functionalities are incorporated and shown to be highly effective.



Hybrid Arrays: How Many RF Chains are Required to Prevent Beam Squint?

September 2024

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4 Reads

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2 Citations

IEEE Transactions on Wireless Communications

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 determined by only a few physical parameters—bandwidth, array size, and beamforming direction—and can be expressed in a remarkably simple closed form. The result is robust, holding also for suboptimum yet highly appealing beamspace architectures.


Spatially Consistent Air-to-Ground Channel Modeling via Generative Neural Networks

April 2024

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23 Reads

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3 Citations

IEEE Wireless Communications Letters

Amedeo Giuliani

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This article proposes a generative neural network architecture for spatially consistent air-to-ground channel modeling. The approach considers the trajectories of uncrewed aerial vehicles along typical urban paths, capturing spatial dependencies within received signal strength (RSS) sequences from multiple cellular base stations (gNBs). Through the incorporation of conditioning data, the model accurately discriminates between gNBs and drives the correlation matrix distance between real and generated sequences to minimal values. This enables evaluating performance and mobility management metrics with spatially (and by extension temporally) consistent RSS values, rather than independent snapshots. For some tasks underpinned by these metrics, say handovers, consistency is essential.


Parabolic Wavefront Model for Line-of-Sight MIMO Channels

November 2023

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19 Reads

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7 Citations

IEEE Transactions on Wireless Communications

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, and their significance to LOS MIMO communication is substantiated.



Figure 1. Fresnel reflection coefficient (magnitude) as a function of θ i for various refractive indices.
Figure 3. ULAs separated by D and equipped with N t = N r = 8 antennas with spacing d. Arrays have arbitrary orientations ϑ t and ϑ r with respect to the x-axis. The clear and solid circles at source and receiver indicate the antennas and their projections onto the x-axis, respectively. Antennas are connected either via a LOS or a reflected channel off a z-oriented surface. We denote by θ 0 the angle formed by the surface normal and the geometrical path connecting the centroids of the image source and receiver.
Figure 4. Normalized channel eigenvalues for various materials. Parallel ULAs separated by D = 10 m with spacing d(D) in (35).
Figure 5. Spectral efficiency as a function of SNR for various materials. Parallel ULAs separated by D = 10 m with spacing d(D, SNR) in (38).
Figure 6. Normalized channel eigenvalues for various materials. Parallel ULAs separated by D = 10 m with spacing d(D) for the LOS channel and d(D e ) for the reflected channel.

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Wide-Aperture MIMO via Reflection off a Smooth Surface
  • Preprint
  • File available

August 2023

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40 Reads

This paper provides a deterministic channel model for a scenario where wireless connectivity is established through a reflection off a smooth planar surface of an infinite extent. The developed model is rigorously built upon the physics of wave propagation and is as precise as tight are the unboundedness and smoothness assumptions on the surface. This model allows establishing how line-of-sight multiantenna communication is altered by a reflection off an electrically large surface, a situation of high interest for mmWave and terahertz frequencies.

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Wide-Aperture MIMO via Reflection off a Smooth Surface

August 2023

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16 Reads

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14 Citations

IEEE Transactions on Wireless Communications

This paper provides a deterministic channel model for a scenario where wireless connectivity is established through a reflection off a smooth planar surface of an infinite extent. The developed model is rigorously built upon the physics of wave propagation and is as precise as tight are the unboundedness and smoothness assumptions on the surface. This model allows establishing how line-of-sight multiantenna communication is altered by a reflection off an electrically large surface, a situation of high interest for mmWave and terahertz frequencies.



Precoding and Reception for ULA-Based Wide-Aperture MIMO

April 2023

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8 Reads

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1 Citation

IEEE Wireless Communications Letters

An approach is presented to compute the capacity-achieving precoder and receiver for wide-aperture MIMO with uniform linear arrays in O(Nmin2){\mathcal {O}}(N^{2}_{\textsf {min}}) operations as opposed to the O(Nmin2Nmax){\mathcal {O}}(N^{2}_{\textsf {min}} {N_{\textsf {max}}}) required by the singular-value decomposition of the channel matrix; Nmin{N_{\textsf {min}}} and Nmax{N_{\textsf {max}}} are the smallest and largest of the numbers of transmit and receive antennas. This hefty reduction in complexity comes at no cost in performance provided a parabolic wavefront model applies over the arrays, which is the case if the array apertures are not overly large relative to the range. Then, as the number of antennas grows larger, the proposed approach evolves into DFT-based precoders and receivers that are even more easily computable.


Citations (70)


... The phased array radars for imaging and high data rate communication systems require large instantaneous bandwidth and suffer from beam squint and pulse stretching due to narrowband phase shifters employed for beam scanning [14], [15]. The wideband TTD lines, instead of narrowband phase shifters, offer squintfree beam steering over a wide instantaneous bandwidth and large scan volume for Radar, massive multiuser multipleinput multiple-output (MU-MIMO) systems, and wideband THz communication applications [16]- [22]. A typical Xband 10 cm resolution long-range imaging radar requires a time delay of a few nanoseconds for scan coverage of 60 0 in both planes of AESA with 1000 elements [23]. ...

Reference:

X-band 16-Channel Transmit-Receive Plank Unit for High-Resolution Imaging RADAR
Hybrid Arrays: How Many RF Chains are Required to Prevent Beam Squint?
  • Citing Article
  • September 2024

IEEE Transactions on Wireless Communications

... To address this, innovative antenna configurations like Reflectarrays and Reflective Intelligent Surfaces (RIS) have been explored [3]- [5]. RIS, in particular, has been studied to modify wireless channels, but its effectiveness in the far field is limited by signal strength unless the RIS size is impractically large, even for indoor applications [6]. ...

RIS in Indoor Environments: Benchmarking Against Ambient Propagation
  • Citing Conference Paper
  • October 2023

... In traditional cellular networks, serving cells are typically selected based on metrics such as RSRP. While this metric may be suitable for gUEs, it often falls short for UAVs closely packed along the AH, where the high channel correlation, driven by dominant LoS conditions [29], can severely affect network performance. To optimally determine the cells designated to serve UAVs along the pre-defined AH, we now introduce a novel metric that captures the multiplexing capability, average channel quality gain, and interference. ...

Spatially Consistent Air-to-Ground Channel Modeling via Generative Neural Networks
  • Citing Article
  • April 2024

IEEE Wireless Communications Letters

... In [19], the spherical wavefront was exploited to design the uniform linear arrays for efficient performance and is compared with conventional half-wavelength spaced arrays. The accuracy and relevance of the parabolic model in near-field MIMO is investigated in [20]. The results were extended to the case of uniform rectangular arrays (URAs) also termed two-dimensional arrays in [21], [22]. ...

Validity of the Parabolic Wavefront Model for Near-Field MIMO
  • Citing Conference Paper
  • May 2023

... Furthermore, other custom narrowband signals for IoT or other low-power coverage-limited applications could be developed for 6G that have low PAPR or high resiliency to hardware imperfections for use cases in which spectral efficiency is not the primary concern [32]. These too can be fit into the OFDMA framework by allocating a portion of the band. ...

Reference:

6G Takes Shape
Spectral vs Energy Efficiency in 6 G: Impact of the Receiver Front-End

IEEE BITS the Information Theory Magazine

... By contrast, the phase-shift differences between the antennas cannot generally be neglected since these are relative to the wavelength. However, the exact expression can be simplified using the Fresnel approximation [31] (also known as the paraxial approximation [32]). In particular, the distance between the transmit and receive antennas can be approximated by using the first-order Taylor series expansion as ...

Parabolic Wavefront Model for Line-of-Sight MIMO Channels
  • Citing Article
  • November 2023

IEEE Transactions on Wireless Communications

... For IRS-aided MU communication network, DoF has been well investigated by many researchers [32]. In [33], it was proved that as IRS aperture increased, DoF increased, which did not need the growth of transmitter, receiver and multipath propagation. A wiretap channel of an IRS-aided secure MIMO wireless network was considered in [34], where the lower and upper bounds of the secure DoF were respectively derived. ...

DOF Augmentation via IRS for Line-of-Sight Communication
  • Citing Conference Paper
  • October 2022

... Different studies highlight the importance of dedicated BS deployments, interference and handover management for high altitude UAVs operating as user equipment (UE) [9]- [11]. The field trails in [9] demonstrate a strong uplink (UL) interference over ground users with the increase in UAVs altitude levels. ...

Coexistence of UAVs and Terrestrial Users in Millimeter-Wave Urban Networks

... With the increasing of the number of antennas, fully digital architecture is costly in terms of hardware implementation and energy consumption. To overcome the issue, hybrid analog/digital architectures have been extensively studied in massive MIMO communication systems [22], [23]. The key idea of hybrid architecture is to partition the signal processing into analog and digital domains, which are realized by analog phase shifters and baseband digital signal processing with few RF chains. ...

Wide-Aperture MIMO via Reflection off a Smooth Surface
  • Citing Article
  • August 2023

IEEE Transactions on Wireless Communications

... It is often considered that the direct link between the base station (BS) and users is not available. For instance, in [16], the RIS-aided communication system was handled through MIMO transmission by considering an upper limit for the channel capacity of the RIS for the blocked direct link scenario. In [4], the hybrid beamforming design was proposed in the RIS-assisted point-to-point MIMO system without the consideration of the direct link. ...

Line-of-Sight MIMO via Intelligent Reflecting Surface
  • Citing Article
  • January 2022

IEEE Transactions on Wireless Communications