RCS measurement of small circular holes

The Rayleigh scattering from small circular holes in a thin screen was studied. The backscattered fields calculated from a dipole expansion were compared those from an exact solution and measurements for a single, electrically small hole. The backscatter from the holes is given by rather simple expansion where the first term is adequate for a good engineering solution. These expressions are within 1 dB of the exact solution for ka⩽0.96 for E-plane scattering and ka⩽0.69 for H-plane scattering when a is the hole's radius and k is the propagation constant     

On the three-coefficient window family

The definition of the three-coefficient window family is extended to the form: w(t,b)=0.5(1-b)+0.5 cos(2πt/T)+0.5b cos(4πt/T ), |t|⩽T/2, -0.25⩽b⩽0.25, and its spectral behavior is investigated. It is found that with the parameter b adjusted to different values, this window family can attain a minimum main lobe width of 1.38, a maximum sidelobe attenuation of 64.19 dB, and a highest cutoff rate of 30 dB/oct     

Binary [18,11]2 codes do not exist-Nor do [64,53]2 codes

A binary, linear block code C with block length n and dimension n is commonly denoted by [n, k] or, if its minimum distance is d, by [n, k,d]. The code's covering radius r(C) can be defined as the smallest number r such that any binary column vector of length (n-k) can be written as a sum of r or fewer columns of a parity-check matrix of C. An [n,k] code with covering radius r is denoted by [n,k]r. R.A. Brualdi et al., (1989) showed that l(m,r) is defined to be the smallest n such that an [n,n-m]r code exists. l(m,2) is known for m⩽6, while it is shown by Brualdi et al. that 17⩽l(7,2)⩽19. This lower bound is improved by A.R. Calderbank et al. (1988), where it is shown that [17,10]2 codes do not exist. The nonexistence of [18,11]2 codes is proved, so that l(7,2)=19. l[7.2)=19 is established by showing that [18,11]2 codes do not exist. It is also shown that [64,53]2 codes do not exist, implying that l(11,2)⩾65     

Evidence for continuous visible chemical lasing from the fast nearresonant energy transfer pumping of atomic sodium

Energy transfer from selectively formed metastable states of SiO is used to pump sodium-atom laser amplifiers at λ≈569 nm (4d²D-3p²P), λ≈616 nm (5s²S-3p² P), and λ≈819 nm (3d²D -3p²P). The a³Σ⁺ and b³Π states of SiO are generated in high yield from the Si+N₂O→SiO+N₂ reaction. The energy stored in the triplet states is transferred in a highly efficient collisional process to pump sodium atoms to their lowest excited 3d² D, 4d²D, and 5s²S states. Adopting a sequence in which high concentrations of silicon and sodium atoms are mixed and oxidized, a continuous amplification (gain condition) is monitored which suggests the creation of a population inversion among the receptor sodium-atom energy levels and forms the basis for full cavity oscillation on the Na4 d²D-3p²P transition at 569 nm     

Constructions of error-correcting DC-free block codes

Two DC-free codes are presented with distance 2d, b ⩾1 length 2n+2r(d-1) for d⩽3 and length 2n+2r(d-1)(2d -1) for d>3, where r is the least integer ⩾log₂ (2n+1). For the first code l=4, c=2, and the asymptotic rate of this code is 0.7925. For the second code l=6, c=3, and the asymptotic rate of this code is 0.8858. Asymptotically, these rates achieve the channel capacity. For small values of n these codes do not achieve the best rate. As an example of codes of short length with good rate, the author presents a (30, 10, 6, 4) DC-free block code with 2²¹ codewords. A construction is presented for which from a given code C ₁ of length n, even weight, and distance 4, the author obtains a (4n, l, c, 4) DC-free block code C₂, where l is 4, 5 or 6, and c is not greater than n+1 (but usually significantly smaller). The codes obtained by this method have good rates for small lengths. The encoding and decoding procedures for all the codes are discussed     

Queueing analyses of traffic access control strategies withpreemptive and nonpreemptive disciplines in wideband integrated networks

A multiserver queueing model of access control strategies for a wideband integrated services digital network (ISDN) is considered. There are two types of service requests (SRs): Type 1 SR requires b servers (basic bandwidth units, BBUs) of the c available servers, while Type 2 SR requires a single server. Both types of SRs are queuable in two separate infinite-size buffers. A Type 1 SR is allowed to seize d(⩽b) servers at first and then acquire additional servers as they become available to meet the original requirement of b servers. The arrival processes of both types of SRs are Poisson and the service times are exponentially distributed with mean b/dμ₁ and 1/μ₂ for Type 1 SR and Type 2 SR, respectively. Preemptive and nonpreemptive priority disciplines with movable boundary are analyzed using the Neuts' matrix-analytic approach. Numerical examples of the queue length distribution and the mean waiting time of the SRs are presented     

New minimum distance bounds for certain binary linear codes

Let an [n, k, d]-code denote a binary linear code of length n, dimension k, and minimum distance at least d. Define d(n, k) as the maximum value of d for which there exists a binary linear [n, k, d]-code. T. Verhoeff (1989) has provided an updated table of bounds on d(n, k) for 1⩽k⩽n⩽127. The authors improve on some of the upper bounds given in that table by proving the nonexistence of codes with certain parameters     

Short codes with a given covering radius

The covering radius r of a code is the maximum distance from any vector in the space containing the code to the nearest codeword. The authors introduce a new function l(m,r), called the length function, which equals the smallest length of a binary code of codimension m and covering radius r. They investigate basic properties of the length function. Projective geometries over larger fields are used to construct families of codes which improve significantly the upper bound for l(m,2) obtained by amalgamation of Hamming codes. General methods are developed for ruling out the existence of codes of covering radius 2 with a given codimension and length resulting in lower bounds for l(m,2). A table is presented which gives the best results now known for l(m,r) with m⩽12 and r⩽12     

TE10 mode scattering by a rectangular resistive film ofarbitrary dimensions placed along the rectangular waveguides axis

Transverse electric (TE)₁₀ mode scattering by a resistive film of arbitrary width (d) and length (l) placed in the longitudinal section of a rectangular waveguide parallel to its narrow faces is investigated. The vector integral equation for the discontinuity of the tangential magnetic field on the film is formulated. The equation is solved by the Galerkin method using basis functions, each of them taking into account the discontinuity behavior near the film edge. For a film that is sufficiently short (l≪d), approximate expressions for the scattering matrix elements are obtained. The scattering matrix for a wide range of values of the film width, length, surface impedance, and frequency is calculated     

The performance of depressed-cladding single-mode fibers withdifferent b/a ratios

Depressed-cladding single-mode fibers with a first cladding-to-core ratio (b/a) ranging from seven to one and relative refractive-index differences of the core from 0.25 to 0.35% and of the cladding from 0 to -0.2% are investigated experimentally with respect to the LP₁₁ mode cutoff properties, the mode-field diameter, the dispersion, and the bending attenuation. Numerical calculations based on actual profile parameters of the bend loss and the mode-field diameter agree well with experiment. It is demonstrated that low attenuation and bend losses independent of b/a can be obtained if the cutoff wavelength is kept constant by adjusting the core radius, the core index or the index of the first cladding     

A proposed chemically pumped laser based on theb→X transition in O2

Simple kinetics calculations demonstrate that the well-known electronic energy pooling reaction involving O₂(a ¹Δ) is capable of producing an effective population inversion in O₂(b¹Σ). The densities of O₂(a¹Δ) which are potentially achievable suggest that the extractable energy storage density of an O ₂(b→X) chemical laser might exceed 0.5 MJ/m³. The b→X emission lifetime measurements conducted under conditions of high relative O₂( b¹Σ) density reveal no evidence of rapid self-quenching effects which would be potentially detrimental to laser performance. The relatively long energy storage times predicted, together with freedom from reagent mixing requirements, make such a laser an attractive alternative to other existing and proposed short wavelength chemical lasers     

Even-mode characteristics of the bilateral slotline

Transmission line data for the even-mode characteristics of a bilateral slotline (double-sided slotline) are presented. The slotline is analyzed by the spectral Galerkin method, and results are presented for the slot wavelength and characteristic impedance for substrates having ε_r=2.22, 3.5, 6.0, 9.8, and 12.8. The results are presented for 0.01⩽√ε_r-1d/λ ₀⩽0.25, where 2d is the substrate thickness and ε_r its dielectric constant     

Balancing sets of vectors

For n>0, d⩾0, n≡d (mod 2), let K(n, d) denote the minimal cardinality of a family V of ±1 vectors of dimension n, such that for any ±1 vector w of dimension n there is a v∈V such that |v- w|⩽d, where v-w is the usual scalar product of v and w. A generalization of a simple construction due to D.E. Knuth (1986) shows that K(n , d)⩽[n/(d+1)]. A linear algebra proof is given here that this construction is optimal, so that K(n, d)-[n/(d+1)] for all n≡d (mod 2). This construction and its extensions have applications to communication theory, especially to the construction of signal sets for optical data links     

New data for shielding by openwork structure

The authors provide measured data for comparison with the prediction of Chen (IEEE Trans. on Microwave Theory Tech., vol.MTT-21, p.1-6, 1973) on the insertion loss of openwork structures. It is shown that the validity of Chen's formulae extends beyond the stated range of d/λ<0.5 to d/λ<1.0, where d /λ is the ratio of aperture spacing to wavelength     

Proposal for a novel wiggler

A magnetostatic-field configuration that is independent of the axial (z) coordinate is presented. The motion of an electron in the xz plane in this magnetostatic field is determined. The velocities and the orbit of the electron are identical to those in the ordinary FEL (free electron laser) but with other expressions for the parameters ω₀ and a_ω. An expression for the wavelength of the radiation of the fundamental harmonic (λ_s) is derived. Some numeric examples of λ_s and a_ω are given, and it is concluded that the method presented is promising for obtaining high values of a_ω     

A limit theorem for n-phase Barker sequences

It is proved that for 3⩽L⩽19, except for L =6, the total number of normalized n-phase Barker sequences of length L increases without limit as n goes to infinity     

On linear structure and phase rotation invariant properties ofblock M-PSK modulation codes

Two important structural properties of block M(=2^' )-ary PSK modulation codes, linear structure and phase symmetry, are investigated. An M-ary modulation code is first represented as a code with symbols from the integer group S_M-PSK=(0,1,2,---,M-1) under modulo-M addition. Then the linear structure of block M-PSK modulation codes over S_M-PSK with respect to modulo- M vector addition is defined, and conditions are derived under which a block M-PSK modulation code is linear. Once the linear structure is developed, the phase symmetry of block M-PSK modulation codes is studied. In particular, a necessary and sufficient condition for a block M-PSK modulation code that is linear as a binary code to be invariant under 2^h/180°M phase rotation, for 1⩽h⩽l is derived. Finally, a list of short 8-PSK and 16-PSK modulation codes is given, together with their linear structure and the smallest phase rotation for which a code is invariant     

The (d,k) subcode of a linear block code

A simple technique employing linear block codes to construct (d,k) error-correcting block codes is considered. This scheme allows asymptotically reliable transmission at rate R over a BSC channel with capacity C_BSC provided R ⩽C_d,k-(1+C_BSC), where C_d,k is the maximum entropy of a (d,k ) source. For the same error-correcting capability, the loss in code rate incurred by a multiple-error correcting (d,k) code resulting from this scheme is no greater than that incurred by the parent linear block code. The single-error correcting code is asymptotically optimal. A modification allows the correction of single bit-shaft errors as well. Decoding can be accomplished using off-the-shelf decoders. A systematic (but suboptimal) encoding scheme and detailed case studies are provided     

A strengthening of the Assmus-Mattson theorem

Let w₁=d,w₂,…,w _s be the weights of the nonzero codewords in a binary linear [n,k,d] code C, and let w' ₁, w'₂, …, w'₃, be the nonzero weights in the dual code C1. Let t be an integer in the range 0<t<d such that there are at most d-t weights w'_i with 0<w'_i ⩽n-t E. F. Assmus and H. F. Mattson, Jr. (1969) proved that the words of any weight w_i in C form a t-design. The authors show that if w₂⩾d+4 then either the words of any nonzero weight w_i form a (t+1)-design or else the codewords of minimal weight d form a {1,2,…,t,t+2}-design. If in addition C is self-dual with all weights divisible by 4 then the codewords of any given weight w_i form either a (t +1)-design or a {1,2,…,t,t+2}-design. The proof avoids the use of modular forms     

A (16, 9, 6, 5, 4) error-correcting DC free block code

A (2n, k, l, c, d) DC free binary block code is a code of length 2n, constant weight n, 2^k codewords, maximum runlength of a symbol l , maximum accumulated charge c, and minimum distance d . The purpose of this code is to achieve DC freeness and error correction at the same time. The goal is to keep the rate k/2 n and d large and l and c small. Of course, these are conflicting goals. H.C. Ferreira (IEEE Trans. Magn., vol.MAG-20, no.5, p.881-3, 1984) presented a (16, 8, 8, 5, 4) DC free code. Here, a (16, 9, 6, 5, 4) DC free code is presented. Easy encoding and decoding algorithms are also given