Analysis of the Propagation in High-Speed Interconnects for MIMICs by Means of the Method of Analytical Preconditioning: A New Highly Efﬁcient Evaluation of the Coefﬁcient Matrix

: The method of analytical preconditioning combines the discretization and the analytical regularization of a singular integral equation in a single step. In a recent paper by the author, such a method has been applied to a spectral domain integral equation formulation devised to analyze the propagation in polygonal cross-section microstrip lines, which are widely used as high-speed interconnects in monolithic microwave and millimeter waves integrated circuits. By choosing analytically Fourier transformable expansion functions reconstructing the behavior of the ﬁelds on the wedges, fast convergence is achieved, and the convolution integrals are expressed in closed form. However, the coefﬁcient matrix elements are one-dimensional improper integrals of oscillating and, in the worst cases, slowly decaying functions. In this paper, a novel technique for the efﬁcient evaluation of such kind of integrals is proposed. By means of a procedure based on Cauchy integral theorem, the general coefﬁcient matrix element is written as a linear combination of fast converging integrals. As shown in the numerical results section, the proposed technique always outperforms the analytical asymptotic acceleration technique, especially when highly accurate solutions are required.


Introduction
The latest advances in the development of integrated circuit technology have increased the attractivity of microstrip transmission lines for the realization of high-speed interconnects in monolithic microwave and millimeter waves integrated circuits (MIMICs). In this contest, the width and the thickness of the metallic region can be comparable, and the transverse cross-section is closed to a trapezoidal shape due to the etching undercuts, or because of the electrolytical growth process. For all these reasons, the classical approaches, based on the modeling of the metallic region as a zero-thickness flat strip, are inappropriate for an accurate analysis of the propagation. Great attention has been paid in the last few decades to developing techniques which are able to evaluate the effect of the thickness and the cross-sectional shape of the metallic strip on the propagation [1][2][3][4][5][6][7][8][9][10]. Among others, integral equation formulations have been widely preferred.
In order to search for the solution of an integral equation which cannot be expressed in a closed form, a discretization scheme has to be provided. On the other hand, it is well-known that a direct discretization can lead to ill-conditioned and/or dense matrices for which the convergence of the approximate solution to the exact one, if it exists, cannot be guaranteed. A possible way to overcome this problem is the use of compression techniques [11,12], the Nyström method [13,14], or the direct formulation of the problem in terms of a Fredholm second-kind integral equation [15], just to give some examples.
In a recent paper [16], the analysis of bound and leaky modes propagation in single and coupled polygonal cross-section microstrip lines was carried out by generalizing the techniques presented in Refs. [17][18][19][20][21][22][23][24][25][26][27][28][29][30] for the analysis of propagation, radiation and scattering problems. The problem has been formulated as a homogeneous one-dimensional electric field integral equation (EFIE) in the spectral domain. A converging matrix equation has been achieved by means of the method of analytical preconditioning (MAP) [31], i.e., the Galerkin method, with suitable expansion functions such that Steinberg's theorems can be applied [32]. In Ref. [16], this goal was reached by selecting expansion functions with a closed form Fourier transform reconstructing the physical behavior of the unknowns, even on the wedges. As a result, just a few expansion functions are enough to reconstruct the solution with good accuracy. Moreover, the Galerkin projection integrals have been written in closed form, thus leading to coefficient matrix elements expressed as combinations of one-dimensional integrals over the selected inverse Fourier transform integration path.
Unfortunately, the numerical evaluation of the coefficient matrix elements is burdensome. Since the expansion functions are continuous functions defined on finite supports, their spectral domain counterparts are oscillating functions with an algebraic asymptotic decay along the inverse Fourier transform integration path. For this reason, the coefficient matrix elements are improper integrals whose integrands are oscillating functions, which can have a slow asymptotic decay. As such, the simulations can be time-consuming when highly accurate solutions are searched for. A classical way to overcome this problem is the extraction of suitable asymptotic contributions from the integrands, such that the integrals of the extracted contributions can be expressed in closed form [33][34][35]. Unfortunately, such a technique leads to accelerated integrands with a faster but still algebraic decay, and it does not modify the asymptotic oscillating behavior of the integrands themselves. As a result, a slower and slower numerical convergence is expected as the accuracy required for the solution increases.
Recently, a different perspective was proposed in Refs. [36][37][38][39][40][41][42][43]. Instead of trying to speed up the convergence of the slowly converging integrals detailed above, fastconverging alternative expressions have been individuated. This can be done by means of a suitable application of the Cauchy integral theorem, which takes advantage of the mathematical properties of the integrands. Even if a general and common line of reasoning can be individuated for all the procedures presented in these papers, each of them is devised for the solution of a specific problem.
Following the same line of reasoning, in this paper, a new fast-converging expression for the improper integrals of oscillating and slowly decaying functions, arising from the analysis of the propagation in polygonal cross-section microstrip lines when an EFIE formulation in the spectral domain is discretized by means of MAP, is provided. A brief overview of the formulation of the problem and the discretization technique adopted in Ref. [16] is presented in Section 2. Section 3 is devoted to the fast evaluation of the coefficient matrix elements: the general improper integral, whose integrand involves the product of two confluent hypergeometric functions of the first kind (CHF1), is expressed as a linear combination of proper integrals and improper integrals over integration paths, along which the integrands, involving the products of two confluent hypergeometric functions of the second kind (CHF2), are non-oscillating functions. The comparisons between the presented technique and the analytical asymptotic acceleration technique (adopted in Ref. [16]) provided in Section 4 clearly state that the technique proposed in this paper always outperforms the analytical asymptotic acceleration technique, especially when highly accurate results are required. Figure 1 shows the geometry of the problem in hand: a perfectly electrically conducting (PEC) polygonal cross-section strip in a homogeneous and isotropic half-space (medium 2) of relative dielectric permittivity ε r2 placed on a grounded planar layer (medium 1) of relative dielectric permittivity ε r1 > ε r2 and thickness d. Thus, k l = k 0 √ ε rl with l ∈ { 1, 2} denotes the wavenumber in medium l, where k 0 = ω √ ε 0 µ 0 = 2π/λ 0 is the free-space wavenumber in vacuum, ω is the angular frequency, ε 0 and µ 0 are the dielectric permittivity and the magnetic permeability of vacuum, respectively, and λ 0 is the wavelength in vacuum.  The analysis of the propagation needs to assume a behavior of the fields along the y axis of the kind

Background: Formulation of the Problem and Matrix Equation
is the mode propagation constant. According to the procedure detailed in Ref. [16], the field can be written as the superposition of the fields generated by the surface current densities on each side of the considered strip. Moreover, remembering that the edge behavior of the components of the surface current density on the i-th side is [44] ( ) ( ) , and that the transverse component of the surface current density is continuous even on the wedges, i.e., where q and i denote two consecutive sides, the following spectral domain integral representation for the longitudinal and the transverse components of the electric field in the upper half-space can be established [16]: , and the superscripts t and y denote the functional dependence on the transverse current and the longitudinal current, respectively, where Let us introduce a Cartesian coordinate system (x, y, z) with the origin on the ground plane, the y axis parallel to the strip axis and the z axis orthogonal to the interface between the involved media. Moreover, let us number the L sides of the strip clockwise and introduce a local Cartesian coordinate system (x i , y, z i ) on the i-th side, of length 2a i , with the origin at the center of the side itself in the position (x i , z i ) with respect to the main coordinate system, such that the z i axis is orthogonal to the i-th side and oriented in the outward direction, and the angle of the x i axis with respect to the x axis is denoted by ϕ i .
The analysis of the propagation needs to assume a behavior of the fields along the y axis of the kind e −jvy , where v = v R − jv I is the mode propagation constant. According to the procedure detailed in Ref. [16], the field can be written as the superposition of the fields generated by the surface current densities on each side of the considered strip. Moreover, remembering that the edge behavior of the components of the surface current density on the i-th side is [44] with t ± i ≥ 1/2, and that the transverse component of the surface current density is continuous even on the wedges, i.e., C i = J qx q a q = J ix i (−a i ), where q and i denote two consecutive sides, the following spectral domain integral representation for the longitudinal and the transverse components of the electric field in the upper half-space can be established [16]: with s ∈ x j , y , and the superscripts t and y denote the functional dependence on the transverse current and the longitudinal current, respectively, where with r ∈ {t, y}, and P and S denote the primary (free-space) contribution and the scattered contribution (due to the dielectric slab and the ground plane), respectively, C denotes the selected inverse Fourier transform integration path in the complex plane u = u R + ju I , J i (u) is the Fourier transform of the current on the i-th side with respect to x i , , q, i and p denote three consecutive sides, and the kernels are summarized in the Appendix A.
As such, by forcing the tangential components of the electric field to be zero on the strip surface, a homogeneous EFIE in the spectral domain is obtained with s ∈ x j , y and j = 1, 2, . . . L.
In order to guarantee fast convergence, the Galerkin method, with the expansion functions introduced in Ref. [27], reconstructing the physical properties of the fields even on the wedges of the PEC strip, is used to discretize the equation in (5). The Fourier transforms of the selected expansion functions can be expressed in closed form as [27] φ (α,β) where the symbol 1 F 1 (· ; · ; ·) denotes the CHF1 and Γ(·) is the Gamma function [45]. On the basis of this property and the reciprocity theorem, it is not difficult to demonstrate that the convolution integrals resulting from Galerkin projection can be reduced to algebraic products [16]. As such, the elements of the obtained matrix equation can be expressed as linear combinations of one-dimensional integrals over the selected inverse Fourier transform integration path.

Fast Evaluation of the Matrix Coefficients
For n, m ≥ 0, the general integral associated with the primary contribution can be written as where f i u, x j , z j is one of the kernels in Equations (4a)-(4d), T i,j u, x j , z j is defined in Equation (A1f), and the star denotes the complex conjugate of a complex number, while the general integral associated to the scattered contribution is where g i u, x j , z j is one of the kernels in Equations (4e)-(4h), g ∞ i u, x j , z j is obtained starting from g i u, x j , z j by replacing C K (u) for K ∈ {E, H}, defined in Equations (A1m) and (A1n), respectively, with C ∞ Since the coefficient matrix elements for n = −1 and/or m = −1 can be written in a similar way, they are here omitted for the sake of brevity. The integrals in Equations (7), (8b) and (8c) are improper integrals. According to the asymptotic behavior for the large argument of the CHF1 [45], i.e., where the upper sign has to be chosen for −π/2 ≤ arg(u) < 3π/2 and the lower sign for −3π/2 < arg(u) ≤ π/2, and observing that C K (u) − C ∞ K (u) |u|→+∞ ∼ e −2|u|d , it can be concluded that the integrand of the integral in Equation (8b) has an asymptotic exponential decay, while the integrals in Equations (7) and (8c) involve asymptotically oscillating functions with an algebraic decay in the worst cases. As a result, the numerical evaluation of such kinds of integrals is less and less efficient as the accuracy required for the solution is higher. The asymptotic decay of the integrands in Equations (7) and (8c) can be sped up by pulling out the asymptotic behavior of the integrand such that the integrals of the extracted contributions can be expressed in closed form. Nevertheless, such a procedure does not change the oscillating nature of the integrands, which keep on having an algebraic decay. Therefore, the numerical convergence of the accelerated integrals, even if faster, is still strongly dependent on the accuracy required for the solution.
A new procedure, overcoming this problem, is shown in the following. For the sake of brevity, only the integrals associated to the primary contribution for n, m ≥ 0 and the propagation of bound modes are considered, the generalization of the proposed technique to the other cases being straightforward. For bound modes propagation, the propagation constant is real, i.e., v = v R , and such that k 2 < v R < k 1 ; therefore, the square-roots branch points of the free-space Green's function in the spectral domain are imaginary numbers. Let us assume the real axis of the complex plane u as the inverse Fourier transform integration path C.
The properties of the function in Equation (12) can be immediately deduced from the following relation between the CHF1 and the CHF2 [45]: As such, ψ (α,β) n (u) is a many-valued function if α + β is a non-integer number with a branch-cut for u R = 0 and u I > 0, and is singular for u = 0 except when α = β = −1/2 and n = 0 simultaneously.
Let us introduce the auxiliary function: where w, s, t ∈ {0, 1}, t (0) l = α l and t (1) l = β l , with l ∈ {i, j}. Such a function is generally a many-valued function with two extra branch-cuts associated with the involved CHF2. The first one is along the imaginary axis of the complex plane u (u R = 0) with a branch point at u = 0, while the second one lies on the hyperbole described by the equation where s i,j and c i,j are defined in the Appendix A, with a branch point at u = −jsgn(z i (x, z))s i,j v 2 R − k 2 2 . By setting the following new representation for the integral in Equation (7) can be immediately stated n,m,w,s,t (u)du (16) where r > 0 in order to avoid the singularity of F (i,j) n,m,w,s,t (u) for u = 0. The asymptotic behavior for the large argument of the CHF2, i.e., [45] U(a; b; u) |u|→+∞ ∼ u −a (17) for −3π/2 < arg(u) < 3π/2, allows us to obtain the following representation for the function in Equation (14) on its principal sheet: n,m,w,s,t (u) =F n,m,w,s,t (u)e G (i,j) w,s,t (u) (18) whereF (i,j) n,m,w,s,t (u) is a non-oscillating function with an asymptotic algebraic decay of the kind 1/u γ with γ ≥ 2, G where R 2 (u) is defined in Equation (A1r), and it is simple to observe that β where {·} denotes the imaginary part of a complex number and the quantitieŝ have been introduced for the sake of the simplicity of notation, identifies the path along which F where {·} denotes the real part of a complex number.
To conclude, we can simply demonstrate that the general improper integral at the right-hand side of Equation (16) can be expressed as a combination of a proper integral and an improper integral of a non-oscillating function by means of the Cauchy's integral theorem and the Jordan's lemma. Indeed, supposing, just for an example, that 0 ≤ŝ < s i,j , due to the properties of F (i,j) n,m,w,s,t (u) detailed above, it is simple to state that n,m,w,s,t (u)du (22) where C is the dotted gray closed contour defined in Figure 2, C (the black solid line in Figure 2) is the integration path defined by the Equations in (20) and (21)

Numerical Results and Discussion
An approximation of the mode propagation constants can be obtained by searching for the nulls of the determinant of the truncated matrix equation. In Ref. [16], following this line of reasoning, the effectiveness of the discretization scheme presented in Section 2 has been widely demonstrated. Indeed, few expansion functions have been needed to achieve accurate solutions for both bound and leaky mode propagations. As such, in order to avoid unnecessary repetitions, no details are given in this section about the convergence of the method and the comparisons with the literature. Instead, this section is aimed at

Numerical Results and Discussion
An approximation of the mode propagation constants can be obtained by searching for the nulls of the determinant of the truncated matrix equation. In Ref. [16], following this line of reasoning, the effectiveness of the discretization scheme presented in Section 2 has been widely demonstrated. Indeed, few expansion functions have been needed to achieve accurate solutions for both bound and leaky mode propagations. As such, in order to avoid unnecessary repetitions, no details are given in this section about the convergence of the method and the comparisons with the literature. Instead, this section is aimed at showing that the procedure detailed in Section 3 always outperforms the classical analytical asymptotic acceleration technique used in Ref. [16] in terms of computation time.
Let us define the CPU time ratio as the ratio between the computation time needed to fill the coefficient matrix as obtained by using the analytical asymptotic acceleration technique with respect to the technique detailed above. The simulations are performed on a laptop equipped with an Intel Core I7-10510U 1.80 GHz-2.30 GHz, 16 GB RAM, running Windows 10 Home 64 bit and the integrals are evaluated by means of an adaptive Gaussian quadrature routine.
The geometry of the examined problem is sketched in Figure 3a: a general trapezoidal cross-section strip in vacuum (ε r2 = 1) with AB = 3w/4, BC = t, CD = w andB =Ĉ = π/2 on a grounded dielectric layer of thickness d = 0.2117λ 0 and a relative dielectric permittivity ε r1 = 9.8. We assume v R /k 0 = 1.5, w/λ 0 = 0.5, 1.0, 2.0 and t/w = 0.1, 0.5, 1.0 as test cases and set the accuracy of the quadrature routine in such a way as to guarantee the convergence of six significant digits in the numerical evaluation of the integrals. As can be clearly deduced by inspecting Figure 3b-d, where the CPU time ratio is plotted as a function of the number of expansion functions used for each component of the surface current density on each side (N), the technique proposed in this paper always outperforms the classical analytical asymptotic acceleration technique. Moreover, the proposed results clearly show that the CPU time ratio is substantially independent of the size of the strip with respect to the wavelength and of the shape of the strip, thus demonstrating the effectiveness of the proposed technique. It is important to note that the CPU time ratio dramatically increases as the number of expansion functions used becomes higher. At the same time, for a given number of expansion functions used, the CPU time ratio increases as the accuracy set for the quadrature routine becomes higher. Indeed, in spite of what happens for the improper integrals of asymptotically oscillating functions, the computation time of the new representation of the coefficient matrix elements is weakly affected by the accuracy required for the numerical evaluation of the integrals. Just for an example, for w/λ 0 = 1.0, t/w = 0.5 and N = 20, the proposed method is four time faster than the analytical asymptotic acceleration technique (see Figure 3c). However, for the same case, the CPU time ratio becomes 7 when the convergence of 8 significant digits in the numerical evaluation of the integrals is required, and even reaches 18 for 10 significant digits of accuracy. Moreover, it is worth noting that this exponential behavior is confirmed also for the other cases examined in Figure 3b-d. To conclude, the technique presented in this paper is particularly advantageous when highly accurate results are searched for. also for the other cases examined in Figure 3b-d. To conclude, the technique presented in this paper is particularly advantageous when highly accurate results are searched for.

Conclusions
Fast convergence was achieved in Ref. [16] by applying the MAP to an integral equation formulation in the spectral domain for the analysis of the propagation in polygonal cross-section microstrip lines. In this paper, the technique proposed in Ref. [16] has been improved by devising a new expression for the coefficient matrix elements, leading to a drastic reduction in the computation time. It is worth observing that the line of reasoning proposed in this paper is versatile, and can be applied to a wide class of problems formulated in the spectral domain and discretized by means of MAP. On the other hand, a specific procedure has to be conceived for the particular problem in hand, which can be seen as the only limitation to the proposed approach. Particularly attractive is the application of the proposed technique to the analysis of the radiation characteristics of a microstrip antenna array, because the integrands of the integrals of mutual contributions between two antennas oscillate faster as the distance between the antennas increases. Moreover,

Conclusions
Fast convergence was achieved in Ref. [16] by applying the MAP to an integral equation formulation in the spectral domain for the analysis of the propagation in polygonal cross-section microstrip lines. In this paper, the technique proposed in Ref. [16] has been improved by devising a new expression for the coefficient matrix elements, leading to a drastic reduction in the computation time. It is worth observing that the line of reasoning proposed in this paper is versatile, and can be applied to a wide class of problems formulated in the spectral domain and discretized by means of MAP. On the other hand, a specific procedure has to be conceived for the particular problem in hand, which can be seen as the only limitation to the proposed approach. Particularly attractive is the application of the proposed technique to the analysis of the radiation characteristics of a microstrip antenna array, because the integrands of the integrals of mutual contributions between two antennas oscillate faster as the distance between the antennas increases. Moreover, the method can be readily generalized to the analysis of the propagation in polygonal cross-section optical waveguides.

Data Availability Statement:
The data presented in this study have been obtained by means of an in-house software code implementing the proposed method.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.