This paper shows that the recently proposed ι -μ shadowed fading model includes, besides the ι -μ model, the η-μ fading model as a particular case. This has important relevance in practice, as it allows for the unification of these popular fading distributions through a more general, yet equally tractable, model. The convenience of new underlying physical models is discussed. Then, we derive simple and novel closed-form expressions for the asymptotic ergodic capacity in ι -μ shadowed fading channels, which illustrate the effects of the different fading parameters on the system performance. By exploiting the unification here unveiled, the asymptotic capacity expressions for the ι -μ, η-μ, and Rician shadowed fading models are also obtained in closed form as special cases.

We present a general and tractable fading model for line-of-sight (LOS) scenarios, which is based on the product of two independent and non-identically distributed κ-μ shadowed power envelopes. Simple closed-form expressions for the probability density function, cumulative distribution function and moment-generating function are derived, which are as tractable as the corresponding expressions derived from a product of Nakagami-m random variables. This model simplifies the challenging characterization of LOS product channels, as well as combinations of LOS channels with non-LOS ones. We leverage these results to analyze performance measures of interest in the contexts of wireless powered and backscatter communications, where both forward and reverse links are inherently of LOS nature, as well as in device-to-device communications subject to composite fading. In these contexts, the proposed model shows a higher flexibility when fitting field measurements with respect to conventional approaches based on product distributions with deterministic LOS, together with a more complete physical interpretation of the underlying propagation characteristics.

In this paper, a novel frequency-locked loop (FLL) architecture for phase-noise reduction of wideband voltage-controlled oscillators (VCO) is presented. The circuit makes use of a phase-shifterless frequency discriminator, which completely avoids the use of any control circuitry to keep the loop in its quadrature condition, highly simplifying the loop. The FLL circuit, which can be operated in a completely transparent way, can be easily interchangeable with the original VCO. An FLL prototype, based on a commercial state-of-the-art VCO, has been fabricated and measured exhibiting a phase-noise reduction higher than 9 and 5.5 dB up to 100-kHz and 1-MHz offset from carrier, respectively, in the complete 4.6-9-GHz band. Absolute phase noise results of -106.5 and -124 dBc/Hz at 100-kHz and 1-MHz offset from carrier have been obtained, showing lower phase noise than other wideband FLLs in the literature.

This paper proposes a novel approach to the statistical characterization of non-central complex Gaussian quadratic forms (CGQFs). Its key strategy is the generation of an auxiliary random variable (RV) that replaces the original CGQF and converges in distribution to it. The technique is valid for both definite and indefinite CGQFs and yields simple expressions of the probability density function (PDF) and the cumulative distribution function (CDF) that only involve elementary functions. This overcomes a major limitation of previous approaches, where the complexity of the resulting PDF and CDF does not allow for further analytical derivations. Additionally, the mean square error between the original CGQF and the auxiliary one is provided in a simple closed-form formulation. These new results are then leveraged to analyze the outage probability and the average bit error rate of maximal ratio combining systems over correlated Rician channels.

In this work we propose the utilization of the Butler matrix as a six-port I/Q demodulator. The inherent symmetry of its topology allows to overcome the intrinsic amplitude and phase imbalances of the traditional approaches making it more suitable for the design of high performance six-port networks. To demonstrate the validity of the proposal a Butler matrix has been designed covering the complete UWB band and its results are compared with the traditional approaches.

Spot-size converters (SSCs) are key for efficient coupling of light between waveguides of different sizes. While adiabatic tapers are well suited for small size differences, they become impractically long for expansion factors around ×100, which are often required when coupling integrated waveguides and free-space beams. Evanescent couplers and Bragg deflectors can be used in this scenario, but their operation is inherently limited in bandwidth. Here, we propose a solution based on a parabolic dielectric interface that couples light from a 0.5μm wide waveguide to a 285μm wide waveguide, i.e., an expansion factor of ×570. We experimentally demonstrate an unprecedented bandwidth of more than 380 nm with insertion losses below 0.35 dB. We furthermore provide analytical expressions for the design of such parabolic spot-size converters for arbitrary expansion factors.