Chayanika Bose

Binding energy of impurity states in spherical quantum dots with parabolic confinement

The binding energy of shallow hydrogenic impurities in spherical quantum dots (QDs) with parabolic confinement is calculated, using a variational approach within the effective mass approximation. The binding energy is computed for GaAs QD as a function of the dot size for different impurity positions, and also as a function of the impurity position for different dot sizes. The results show that the impurity binding energy increases with the reduction in the dot dimension. The binding energy is also found to depend on the location of the impurity, and the same is the maximum for the on-center impurity.

Effect of a parabolic potential on the impurity binding energy in spherical quantum dots

The binding energy of a shallow hydrogenic impurity in a spherical quantum dot (QD) with square-well potential (SWP) is calculated using a perturbation method. The same is also calculated assuming the dot to have an isotropic parabolic potential (PP). The calculated binding energies are computed for GaAs QD as a function of the dot size for different impurity positions, and also as a function of the donor position for different dot sizes. The results show that the impurity binding energies increase with a reduction in the dot dimension. The binding energy is also found to depend on the location of the impurity, and the same is maximum for the on-centre impurity. The comparison of the two impurity states show that the binding energy is lower in the case of the parabolic confinement.

Perturbation calculation of impurity states in spherical quantum dots with parabolic confinement

Abstract An attempt is made to derive the impurity binding energies associated with the ground state and a few adjacent excited levels of a shallow hydrogenic impurity in spherical quantum dot (QD) with parabolic confinement, using the perturbation method. The binding energies are computed for GaAs QD as functions of the dot size and the donor position within the QD. The results show that the impurity binding energies decrease with the increase in dot size. The binding energy is also found to strongly depend on the location of the impurity. For s states, the donor binding energy is maximum for an on-centre impurity, while for 0p and 0d states, the binding energy maxima occur for an impurity located off the dot centre.

Perturbation calculation of donor states in a spherical quantum dot

Abstract An attempt is made to derive the donor states associated with the ground and a few higher levels in spherical quantum dot (QD) using the perturbation method. The binding energies are computed for GaAs QD as functions of the dot size and the impurity position. The results show that the impurity binding energies decrease with the increase in dot dimension. The binding energies are also found to show a strong dependence on the location of the impurity within the dot. For s-levels, the donor binding energy is maximum for an on-centre impurity, while for p and d-levels, the binding energy maxima occur for a donor located off the dot centre.

Effect of polarization and self-energy on the ground donor state in the presence of conduction band nonparabolicity in GaAs–(Al,Ga)As spherical quantum dot

Based on the variational technique, the binding energy associated with the ground donor state in the presence of conduction band nonparabolicity, polarization charge, and self-energy is estimated for a finite-barrier quantum dot, with a hydrogenic impurity located at its center. The overall effect of the conduction band nonparabolicity, polarization charge, and self-energy is found to enhance the ground state binding energy, their influences being most prominent in small dots with high barriers.

Performance of Non-coherent MFSK with Selection and Switched Diversity Over Hoyt Fading Channel

A closed-form expression of cumulative distribution function (CDF) of the instantaneous signal to noise ratio (SNR) in Hoyt fading channel is derived. This CDF and associated formulas are then used to find out the error probability of non-coherent M-ary frequency shift keying with multichannel reception. Simple finite-range integral expression for the symbol error probability (SEP) with selection diversity is found through CDF method. Next, closed-form expressions of moment generating functions (MGF) are presented for the switched diversity case and SEP values are calculated using the derived MGFs. Some other performance parameters like, outage probability and average SNR with switched diversity, are provided. In addition, analytic frameworks are presented for calculation of optimum switching thresholds that ensure minimum outage probability or minimum SEP. The analysis is quite general in the sense that it covers switch and stay combining and Rayleigh fading as special cases.

Electric field induced shifts of electronic energy levels in spherical quantum dot

Abstract The influence of an electric field on the electrons confined in semiconductor spherical quantum dots (QDs) was investigated by using the perturbation method. We illustrate the theory for a single GaAs spherical QD. The computed results show that the applied field lowers the electronic energy levels, the lowering being most pronounced for the ground level. The energy levels are also found to be relatively more influenced by the field in a QD of larger size. We prove a spherical QD to be more sensitive to applied field than a quantum cube as is evident from the comparison of the field induced shifts in their ground levels.

BER Performance of Coherent PSK in Rayleigh Fading Channel with Imperfect Phase Estimation

In this paper average bit error rate (BER) of coherent phase shift keying (PSK) modulations with improper phase estimation have been derived. A simple slow flat wireless fading channel obeying Rayleigh distribution is assumed for channel modelling. In addition the channel is also perturbed by additive white Gaussian noise (AWGN). The phase distortions considered in the paper are random, unbiased, i. e. having zero-mean and follows a Gaussian distribution. Analytical BER is calculated for the first two members of the PSK family, namely binary PSK (BPSK) and quaternary PSK (QPSK), through Hermite’s method of integration. Extensive Monte Carlo simulations were also performed to validate the theoretical results. In order to demonstrate the advantages of modulation schemes that do not require phase synchronization, BER performances of BPSK and QPSK with phase error are compared with the corresponding differential PSK modulations (DPSK and DQPSK respectively) of same constellation size.

Effect of imperfect phase synchronization on the error rate performance of MPSK in Rayleigh, Rician and Nakagami fading channels

This paper provides a framework for error rate performance analysis of M-ary phase-shift keying (MPSK) systems with improper phase estimation over some common fading channels obeying Rayleigh, Rician, or Nakagami-m distribution. In addition, the channel is also perturbed by additive white Gaussian noise (AWGN). The phase distortions considered in the paper are random, unbiased, i.e. having zero-mean, and follows Tikhonov distribution. Analytical average symbol error probability (SEP) is calculated through a new efficient method and plotted as a function of signal to noise ratio (SNR) for different values of the modulation order, M. Our approach relies upon knowledge of the moment generating function (MGF) of the received SNR. The resulting expression is an integral over a finite range of integrands containing only elementary functions. Extensive Monte Carlo simulations were performed to validate the theoretical results. Although the SEP can be computed via direct numerical integration, the MGF method performs far better because direct integration suffers from numerical instability and inaccuracy due to the presence of infinite integration limits or may contain complex mathematical functions.

A Markov Selection Split Reservation Protocol for WDM Optical Networks without Wavelength Conversion

In WDM optical networks, during wavelength reservation if two or more requests compete for the same wavelength even when other free wavelengths are available, a collision occurs. An appropriate selection of wavelength is therefore very important to avoid such conflicts. Markov model is very effective to reduce such conflicts by guessing a wavelength in advance. Further, even after successful probing, a request may be blocked because of reservation failure due to the vulnerable period between wavelength probing and actual reservation. To minimize the effect of such vulnerability, splitting the probe process to fork out a reservation from an intermediate node is an efficient solution. So this paper combines Markov-based selection strategy with an adaptive splitting technique to design a Markov-selection split reservation protocol (MSRP*). MSRP* is compared with three other protocols, including Markov-based backward reservation protocol (MBRP). The comparative results show that the blocking probability improves considerably (~ 30% over MBRP in some cases). Thus, the proposed scheme appears quite promising, especially for the delay-relaxed applications, where blocking is very crucial.