Abstract

Reconfigurable intelligent surfaces (RISs) are emerging as a key technology under 6G paradigm, enabling programmable electromagnetic (EM) transformations for controlling wireless propagation. This paper develops a rigorous EM-based model for RIS-aided channels utilizing the wavenumber-spectrum representation of the Helmholtz equation. We formulate the RIS as a spatially varying reflection operator that multiplicatively modifies the incident field in space. Further, by applying a convex hull-based geometric interpretation of the convolution supports, we establish a Brunn-Minkowski-type bound on achievable DoF. Our analysis identifies unusable wavenumber as a primary degradation factor in non-isotropic environments. We propose a spectral centering operation at RIS to mitigate these losses and validate the model through simulations of multipath enrichment, which demonstrate that efficiency degrades with high transverse wavevectors.

Abstract

The advent of 6G is expected to enable many use cases which may rely on accurate knowledge of the location and orientation of user equipment (UE). The conventional localization methods suffer from limitations such as synchronization and high power consumption required for multiple active anchors. This can be mitigated by utilizing a large dimensional passive reconfigurable intelligent surface (RIS). This paper presents a novel low-complexity approach for the estimation of 5D pose (i.e. 3D location and 2D orientation) of a UE in near-field RIS-assisted multiple-input multiple-output (MIMO) systems. The proposed approach exploits the symmetric arrangement of uniform planar array of RIS and uniform linear array of UE to decouple the 5D problem into five 1D sub-problems. Further, we solve these sub-problems using a total least squares ESPRIT inspired approach to obtain closed-form solutions.

Abstract

A conventional push-push frequency doubler (CPFD) is simple to implement but suffers from limited total
efficiency (ηtotal) at millimeter-wave (mm-wave) frequencies due to negative feedback introduced by the gate-drain capacitance (Cgd). To overcome this limitation, a 41-62.4 GHz mm-wave FD is
proposed in which the NMOS CPFD employs cross-coupled gatesource feedback that generates an in-phase gate second-harmonic voltage with the drain's second-harmonic current. This feedback effectively mitigates the inherent Cgd feedback and doubles
the fundamental gate-source voltage, Vgs[f0]. As a result, the drain 2f0 current generated by the nonlinear second-harmonic transconductance gm2,nonlinear of the active core's NMOS is increased, leading to enhanced drain efficiency (ηD) and ηtotal. The proposed FD is designed in a 65 nm CMOS process. Post layout simulations demonstrate a peak ηD of 30.79%, a peak ηtotal of 13.7%, and a peak 2f0 output power of 5.8 dBm at 46 GHz, with a 12 dBm local oscillator (LO) input and 12.66 mW DC power consumption. The FD achieves a 3 dB bandwidth of 41-
62.4 GHz (41.4%) and an efficiency bandwidth of 42.3-56 GHz (27.7%), where ηtotal ≥ 10%.

Abstract

This paper presents an 18.87-24.19 GHz dual-core, dual-mode
voltage-controlled oscillator (VCO) using a multi-tap (MT)
inductor-based 2-port resonator with capacitive coupling
via a mode-switching block. The 2-port resonator introduces
a gate-to-drain phase shift, suppressing flicker phase
noise (PN), while the high-quality factor of the tank
minimizes thermal PN. The mode-switching block enables dual
resonance modes--high-frequency band (HB) and low-frequency
band (LB), achieving a wide frequency tuning range (FTR)
and low PN in both the 1/f^2 and 1/f^3 regions. The
proposed VCO is implemented in 65 nm CMOS, and post-layout
results show a PN of -68.7 dBc/Hz at a 10 kHz offset and
-118.2 dBc/Hz at a 1 MHz offset from an 18.87 GHz carrier
frequency, with a power
consumption of 25.65 mW and an area of 0.154 mm^2. The
peak figure of merit (FoM) is 189.6 dBc/Hz, and the maximum
1/f^3 noise corner frequency is 347 kHz across the FTR.

Abstract

A conventional push-push frequency doubler (FD) is simple to implement but suffers from limited total efficiency ($eta_{text{total}}$) at millimeter-wave (mm-Wave) frequencies due to negative feedback introduced by the gate-drain capacitance ($C_{mathrm{gd}}$). To overcome this limitation, a 42.5-58 GHz mm-Wave FD is proposed, utilizing cross-coupled gate-to-source feedback. In the proposed topology, the push-push NMOS frequency doubler employs cross-coupled gate-to-source feedback, which generates a second-harmonic voltage at the gate that is in phase with the drain's second-harmonic current. This feedback effectively mitigates the inherent feedback caused by $C_{mathrm{gd}}$, and doubles the gate-to-source voltage of the fundamental harmonic, $V_{gs}[f_{0}]$. As a result, the amplitude of the drain $2f_{0}$ current generated by the nonlinear second harmonic transconductance $g_{m2,nonlinear}$ of the NMOS increases, leading to enhanced drain efficiency ($eta_{mathrm{D}}$) and $eta_{text{total}}$. The proposed FD is designed in a 65 nm CMOS process. Post-layout simulations demonstrate a peak $eta_{mathrm{D}}$ of 28.39%, $eta_{text{total}}$ of 13.7%, and a peak $2f_0$ output power of 6.27 dBm at 47.5 GHz, with a 12 dBm local oscillator (LO) input and 14.92 mW DC power consumption. The doubler achieves a 3 dB bandwidth of 42.5-58 GHz $(30.85%)$ and an efficiency bandwidth 43.75-54.66 GHz $(20.4%)$ where $eta_{text{total}} geq 10%$.

Abstract

We consider a reconfigurable intelligent surface
(RIS)-assisted multi-user communication system. For such a system, we aim to optimally select the RIS phase shifts and precoding vectors for maximizing the effective rank of the weighted channel covariance matrix which essentially improves the channel capacity. For a low-complexity transmitter design, we
employ maximum ratio transmission (MRT) and minimum-mean square error (MMSE) precoding schemes along with water-filling algorithm-based power allocation. Further, we show that MRT and MMSE exhibit equivalent performance and become optimal
when the channel effective rank is maximized by optimally configuring the RIS consisting of a large number of elements

Abstract

Reconfigurable intelligent surfaces (RIS) are emerging as a promising technology for 6G networks due to their
ability to shape the radio propagation environment. This ability
helps to overcome propagation challenges, such as high path loss,
absorption loss, etc., that are particularly needed at millimetre
wave (mmWave) bands. Thus, RIS can enhance the capacity
of next-generation mmWave communication networks. However,
one critical aspect in the design of RIS-aided systems is channel
estimation, as it involves estimating two channels (i.e., base stationRIS and RIS-user terminal) separately based on the compound
channel observed by the receiver. In this paper, we address
this problem by proposing an algorithm that estimates both
channels separately by leveraging the advantages of sparsity of
the mmWave channel. Specifically, the proposed algorithm uses
the parallel factor (PARAFAC) decomposition of the tensor, which
is constructed using the received signal matrices observed under
different RIS phase shift configurations. Our numerical analysis
shows that the proposed algorithm provides significantly smaller
normalized mean square error (NMSE) than the widely used
alternating least squares (ALS) algorithm, particularly when the
number of RIS phase shift configurations is smaller than the
number of RIS elements.

Abstract

This paper aims to explore RIS integration in a millimeter wave (mmWave) communication system with low-complexity transceiver architecture under imperfect channel state information (CSI) assump- tion. Motivated by this, we propose a RIS-aided system with a fully analog architecture at the base station (BS). However, to overcome the drawback of single-user transmission due to the single RF chain in the analog architecture, we propose to employ NOMA to enable multi-user transmission. For such a system, we formulate two problems to obtain the joint transmit beamformer, RIS phase shift matrix, and power allocation solutions that maximize sum rate and energy efficiency such that the minimum rate for each user is satisfied. However, both problems are intractable due to 1) the fractional objective, 2) non-convex minimum rate and unit modulus RIS phase shift constraints, and 3) the coupled optimization variables. Hence, we first tackle the fractional objectives of both problems by reformulating the sum rate and energy efficiency maximization problems into equivalent quadratic forms using the quadratic transform. On the other hand, we employ successive convex approximation and the semi-definite relaxation technique to handle the non-convex minimum rate and unit modulus constraint of the RIS phase shifts, respectively. However, the problems remain non-convex due to the coupled optimization variables. Thus, we propose an alternating optimization-based algorithm that iterates over the transmit beamformer, power allocation, and RIS phase shift subproblems. Further, we also show that the quadratic reformulation is equivalent to the weighted mean square error-based reformulation for the case of sum rate maximization problem. Our numerical results show that the proposed RIS-NOMA integrated analog architecture system outperforms the optimally configured fully digital architecture in terms of sum rate at low SNR and energy efficiency for a wide range of SNR while still maintaining low hardware complexity and cost. Finally, we present the numerical performance analysis of the RIS-NOMA integrated low-complexity system for various system configuration parameters.

Abstract

Robust spectrum sensing is crucial for facilitating opportunistic spectrum utilization for secondary users (SU) in the absense of primary users (PU). However, propagation environment factors such as multi-path fading, shadowing, and lack of line of sight (LoS) often adversely affect detection performance. To deal with these issues, this paper focuses on utilizing reconfig- urable intelligent surfaces (RIS) to improve spectrum sensing in the scenario wherein both the multi-path fading and noise are correlated. In particular, to leverage the spatially correlated fading, we propose to use maximum eigenvalue detection (MED) for spectrum sensing. We first derive exact distributions of test statistics, i.e., the largest eigenvalue of the sample covariance matrix, observed under the null and signal present hypothesis. Next, utilizing these results, we present the exact closed-form expressions for the false alarm and detection probabilities. In addition, we also optimally configure the phase shift matrix of RIS such that the mean of the test statistics is maximized, thus improving the detection performance. Our numerical analysis demonstrates that the MED's receiving operating characteristic (ROC) curve improves with increased RIS elements, SNR, and the utilization of statistically optimal configured RIS. Index Terms--Reconfigurable Intelligent Surfaces, Spectrum Sensing, Maximum Eigenvalue Detector, Correlated Fading, e

Abstract

This paper focuses on optimal beamforming to maximize the mean signal-to-noise ratio (SNR)
for a passive reconfigurable intelligent surface (RIS)-aided multiple-input single-output (MISO) downlink
system. We consider a realistic setting where both the direct and indirect (through RIS) links to the user
equipment (UE) experience correlated Rician fading. Such a general fading model is particularly important to
capture the impact of line-of-sight (LoS) and correlated multipath fading that may occur due to the compact
placement of a large number of RIS elements. The assumption of passive RIS imposes the unit modulus
constraint, which makes the beamforming problem non-convex. To tackle this issue, we apply semidefinite
relaxation (SDR) to obtain the optimal phase-shift matrix and propose an iterative algorithm to obtain
a statistically optimal solution for the transmit beamforming vector and RIS-phase shift matrix. Further,
to measure the performance of the proposed beamforming scheme, we analyze key system performance
metrics such as outage probability (OP) and ergodic capacity (EC). Similar to the existing works, the OP
and EC evaluations rely on the numerical computation of the proposed iterative algorithm. However, this
is not conducive to revealing the functional dependence of system performance on key parameters such as
line-of-sight (LoS) components, correlated fading, number of reflecting elements, number of antennas at the
base station (BS), and fading factor. To overcome this major limitation, we derive closed-form expressions
for the optimal beamforming vector and phase shift matrix for various special cases of the above general
fading model. These fixed-point solutions aid in deriving a closed-form solution for OP that further provides
a direct evaluation of EC. These mathematical expressions are then used to gain useful insights into the
system's performance. We analytically establish the fact that correlated fading is more beneficial than the
independent and identically distributed (i.i.d.) case when the LoS components are blocked. Further, we also
analytically demonstrate that the maximum mean SNR improves linearly/quadratically with the number of
RIS elements in the absence/presence of LoS component under i.i.d. fading.