Optical Communication Using Subcarrier PSK Intensity Modulation Through Atmospheric Turbulence Channels

This paper studies optical communications using subcarrier phase shift keying (PSK) intensity modulation through atmospheric turbulence channels. The bit error rate (BER) is derived for optical communication systems employing either on/off key (OOK) or subcarrier PSK intensity modulation. It is shown that at BER = 10^-6 and a scintillation level of sigma = 0.1, an optical communication system employing subcarrier BPSK is 3 dB better than a comparable system using fixed-threshold OOK. When sigma = 0.2, an optical communication system employing subcarrier BPSK achieves a BER = 10^-6 at SNR = 13.7 dB, while the BER of a comparable system employing OOK can never be less than 10^-4. Convolutional codes are discussed for optical communication through atmospheric turbulence channels. Interleaving is employed to overcome memory effect in atmospheric turbulence channels. An upper bound on BER is derived for optical communication systems employing convolutional codes and subcarrier BPSK modulation.     

Thermoelectric quantum-dot superlattices with high ZT

Following the experimentally observed Seebeck coefficient enhancement in PbTe quantum wells in Pb_1−xEu_xTe/PbTe multiple-quantum-well structures which indicated the potential usefulness of low dimensionality, we have investigated the thermoelectric properties of PbSe_xTe_1−x/PbTe quantum-dot superlattices for possible improved thermoelectric materials. We have again found enhancements in Seebeck coefficient and thermoelectric figure of merit (ZT) relative to bulk values, which occur through the various physics and materials science phenomena associated with the quantum-dot structures. To date, we have obtained estimated ZT values approximately double the best bulk PbTe values, with estimated ZT as high as about 0.9 at 300 K.     

Electrical characterization of poly(amide-imide) for application in organic field effect devices

The admittance spectra and current–voltage (I–V) characteristics are reported of metal–insulator–metal (MIM) and metal–insulator–semiconductor (MIS) capacitors employing cross-linked poly(amide–imide) (c-PAI) as the insulator and poly(3-hexylthiophene) (P3HT) as the active semiconductor. The capacitance of the MIM devices are constant in the frequency range from 10 Hz to 100 kHz, with tan δ values as low as 7 × 10⁻³ over most of the range. Except at the lowest voltages, the I–V characteristics are well-described by the Schottky equation for thermal emission of electrons from the electrodes into the insulator. The admittance spectra of the MIS devices displayed a classic Maxwell–Wagner frequency response from which the transverse bulk hole mobility was estimated to be ∼2 × 10⁻⁵ cm² V⁻¹s⁻¹ or ∼5 × 10⁻⁸ cm² V⁻¹s⁻¹ depending on whether or not the surface of the insulator had been treated with hexamethyldisilazane (HMDS) prior to deposition of the P3HT. From the maximum loss observed in admittance-voltage plots, the interface trap density was estimated to be ∼5 × 10¹⁰ cm⁻² eV⁻¹ or ∼9 × 10¹⁰ cm⁻² eV⁻¹ again depending whether or not the insulator was treated with HMDS. We conclude, therefore, that HMDS plays a useful role in promoting order in the P3HT film as well as reducing the density of interface trap states. Although interposing the P3HT layer between the insulator and the gold electrode degrades the insulating properties of the c-PAI, nevertheless, they remain sufficiently good for use in organic electronic devices.     

Performance bounds for multistep prediction-based blindequalization

Blind equalization attempts to remove the interference caused by a communication channel without using any known training sequences. Blind equalizers may be implemented with linear prediction-error filters (PEFs). For many practical channel types, a suitable delay at the output of the equalizer allows for achieving a small estimation error. The delay cannot be controlled with one-step predictors. Consequently, multistep PEF-based algorithms have been suggested as a solution to the problem. The derivation of the existing algorithms is based on the assumption of a noiseless channel, which results in zero-forcing equalization. We consider the effects of additive noise at the output of the multistep PEF. Analytical error bounds for two PEF-based blind equalizers in the presence of noise are derived. The obtained results are verified with simulations. The effect of energy concentration in the channel impulse response on the error bound is also addressed     

Complex resonances of conducting spheres with lossy coatings

The electromagnetic scattering amplitude for metal objects coated with a lossless dielectric exhibits a large number of resonances versus real frequency. These resonances are a function of the object shape and size, coating thickness, and coating electrical properties. Previously, it was shown that for coated spheres and nonspherical bare objects, these resonances can be understood in terms of phase matched circumnavigating surface waves and the objects' complex eigenfrequencies. The effect of dielectric loss in the coating on the complex eigenfrequencies and phase velocities of these surface waves is presented for a metal sphere coated with a uniform homogeneous dielectric coating. It is seen that the positions of the complex resonance frequencies move away from the real frequency axis under the influence of dielectric loss in the coating. The effect of this is shown to correspond to the changes in the backscatter spectrum versus real frequency as computed using a Mie series expansion. The significant difference between the lossless and lossy dielectric-coated sphere cases is seen to be due to a modification of the phase velocities of surface waves, particularly the whispering gallery types, and the increased attenuation of the surface wave modes     

Extrusion of YAG Tubes Shows that Bottom‐up Processing is Not Always Optimal

Liquid‐feed flame spray pyrolysis provides easily dispersed, unaggregated nanopowders with average particle sizes of 20–70 nm depending on the processing conditions. Their chemical compositions can be controlled to ppm levels via control of the initial precursor solution. In this paper, Y₃Al₅O₁₂ composition nanopowders are produced that are atomically mixed but offer a hexagonal crystal structure rather than a YAG structure. Y₂O₃ and δ‐Al₂O₃ nanopowders are also produced and mixed to evaluate reactive sintering. It is shown that nanopowder/polymer mixtures permit the extrusion of tubes that retain their shape on debindering and sintering to ≥95% theoretical density. More importantly, the sintering behavior of hex‐Y₃Al₅O₁₂ is compared with that of tubes formed using 3:5 Y₂O₃:δ‐Al₂O₃ mixtures to test the so‐called bottom‐up paradigm, which suggests that mixing on the finest length scales should provide optimal control of sintering rates, final densities, and grain sizes. Instead, it is found that reactive sintering is faster and offers better control of final grain sizes. Dense sintered tubes are translucent, and dimensional uniformity is maintained from extrusion through sintering.     

Photon-assisted capacitance–voltage study of organic metal–insulator–semiconductor capacitors

The results are reported of a detailed investigation into the photoinduced changes that occur in the capacitance–voltage (C–V) response of an organic metal–insulator–semiconductor (MIS) capacitor based on the organic semiconductor poly(3-hexylthiophene), P3HT. During the forward voltage sweep, the device is driven into deep depletion but stabilizes at a voltage-independent minimum capacitance, C_min, whose value depends on photon energy, light intensity and voltage ramp rate. On reversing the voltage sweep, strong hysteresis is observed owing to a positive shift in the flatband voltage, V_FB, of the device. A theoretical quasi-static model is developed in which it is assumed that electrons photogenerated in the semiconductor depletion region escape geminate recombination following the Onsager model. These electrons then drift to the P3HT/insulator interface where they become deeply trapped thus effecting a positive shift in V_FB. By choosing appropriate values for the only disposable parameter in the model, an excellent fit is obtained to the experimental C_min, from which we extract values for the zero-field quantum yield of photoelectrons in P3HT that are of similar magnitude, 10^?5 to 10^?3, to those previously deduced for π-conjugated polymers from photoconduction measurements. From the observed hysteresis we deduce that the interfacial electron trap density probably exceeds 10¹⁶ m^?2. Evidence is presented suggesting that the ratio of free to trapped electrons at the interface depends on the insulator used for fabricating the device.     

Generating, capturing and processing supply current signatures from analogue macros in mixed-signal ASICs

Transient Response Testing is a powerful test technique for analogue macros in mixed-signal electronic systems which with some enhancement can be particularly useful for testing deeply buried circuit structures. Supply current testing is finding widespread application in the digital domain and its use in the analogue domain may lead to integrated test methodologies for mixed-signal systems. This paper shows that by utilizing both these techniques, and a low-cost test shell, deeply buried analogue macros can be partitioned, tested using Transient Response Testing and the resulting response accurately captured from the total device supply current. It also contains an analysis of the noise on the supply current, due to digital circuit activity during testing, and demonstrates a test response analysis technique which is insensitive to it.     

On-Line Error Detection for Bit-Serial Multipliers in GF(2m)

In this paper error detection is applied to four finite field bit-serial multipliers. It is shown that by using parity prediction, on-line error detection can be incorporated into these multipliers with very low hardware overheads. These hardware overheads are generally independent of m and comprise only a handful of gates, so for large values of m these overheads are particularly low. The fault coverage of the presented structures has been investigated by simulation experiment and shown to range between 90% and 94.3%.