Direct Observation of Carrier Transportation Process in InGaAs/GaAs Multiple Quantum Wells Used for Solar Cells and Photodetectors

The resonant excitation is used to generate photo-excited carriers in quantum wells to observe the process of the carriers transportation by comparing the photoluminescence results between quantum wells with and without a p-n junction. It is observed directly in experiment that most of the photo-excited carriers in quantum wells with a p-n junction escape from quantum wells and form photoeurrent rather than relax to the ground state of the quantum wells. The photo absorption coei^cient of multiple quantum wells is also enhanced by a p-n junction. The results pave a novel way for solar cells and photodetectors making use of low-dimensional structure.

A time domain multiple-CFOs and CIRs estimation algorithm over wireless multimedia sensor networks

Channel parameters estimation in an orthogonal for the receiver station is a multi-dimensional （MD） frequency division multiple access （OFDMA） system optimization problem, because every user node has a separate local oscillator and every transmitter to receiver link has individual carrier frequency offset （CFO） and channel impulse response （CIR） parameters. In order to reduce the computational complexity for MD optimization, a time domain CFOs and CIRs estimation algorithm over the OFDMA based wireless multimedia sensor networks （WMSN） is proposed in this paper. In this algorithm, the receiver station can decouple the signal from every node by correlation based on specially designed training sequences, so that the MD optimization problem is simplified to an 1-D optimal problem. It is proved that the multiple CFOs can be identified from the correlation result using the phase shift of the consecutive training se- quences. Based on the CFOs estimation result, the CIRs can then he estimated according to the minimum mean square error （MMSE） criterion. The theoretic analysis and simulation results show that the proposed algorithm can effectively decouple the signal from different user nodes and the bit error rate （BER） per- formance curves are close to the ideal estimation when the user number is not large.

Characterization of hot carrier cooling and multiple exciton generation dynamics in PbS QDs using an improved transient grating technique

Multiple exciton generation （MEG） dynamics in colloidal PbS quantum dots （QDs） characterized with an im- proved transient grating （TG） technique will be reported. Only one peak soon after optical absorption and a fast decay within 1 ps can be observed in the TG kinetics when the photon energy of the pump light hv is smaller than 2.7Eg （Eg： band gap between LUMO and HOMO in the QDs）, which corresponds to hot carrier cooling. When hv is greater than 2.7Eg, however, after the initial peak, the TG signal decreases first and soon increases, and then a new peak appears at about 2 to 3 ps. The initial peak and the new peak correspond to hot carriers at the higher excited state and MEG at the lowest excited state, respectively. By proposing a theoretical model, we can calculate the hot carrier cooling time constant and MEG occurrence time constant quantitatively. When MEG does not happen for hv smaller than 2.7Eg, hot carrier cools with a time con- stant of 400 fs. When MEG occurs for hv larger than 2.7Eg, hot carrier cools with a time constant as small as 200 fs, while MEG occurs with a time constant of 600 fs. The detailed hot carrier cooling and MEG occurrence dynamics characterized in this work would shed light on the further understanding of MEG mechanism of various type of semiconductor QDs.

Walsh Hadamard Transform Based Transceiver Design for SC-FDMA with Discrete Wavelet Transform

This article proposes a new transceiver design for Single carrier frequency division multiple access(SCFDMA)system based on discrete wavelet transform(DWT). SCFDMA offers almost same structure as Orthogonal frequency division multiple access(OFDMA)with extra advantage of low Peak to Average Power Ratio(PAPR). Moreover,this article also suggests the application of Walsh Hadamard transform(WHT)for linear precoding(LP)to improve the PAPR performance of the system. Supremacy of the proposed transceiver over conventional Fast Fourier transform(FFT)based SCFDMA is shown through simulated results in terms of PAPR,spectral efficiency(SE)and bit error rate(BER).

Using CSMA/CD for Ethernet Passive Optical Network (EPON)

Using CSMA/CD for EPON can eliminate the augmentations, such as multi-point control protocol and point-to-point emulation, added to the existing 802.3 architecture due to the incompatibility of PON to Ethernet. Both full-duplex EPON system and half-duplex EPON system using CSMA/CD were proposed. In the full-duplex EPON, CSMA/CD is used as the upstream MAC protocol. In the half-duplex EPON system, both upstream and downstream traffic contend for the optical channel through CSMA/CD protocol. The upstream lightwave redirection and collision detection techniques were given. By the analysis and simulation, the throughput performance of the half-duplex EPON system is proven to be as well as that of the existing high speed half-duplex Ethernet LAN.

Modeling and Simulation of P-Aloha,CSMA /CA and MACAW Protocols for Underwater Acoustic Channel

Medium access control( MAC) protocol of underwater acoustic communication network is a key technology for underwater acoustic networks( UANs). Most of the MAC protocols for wireless terrestrial communication networks have been designed with negligible propagation delay. If it is deployed directly in an underwater environment,the UANs will perform inefficiently. In this paper,the characteristics of underwater acoustic channel are modeled and simulated by using the OPNET simulation tool,which are the speed of sound, propagation loss, and four sources for ambient noise: the turbulence,shipping,wind driven waves and thermal noise. The performance of pure Aloha( P-Aloha),carrier sense multiple access with collision avoidance( CSMA / CA) and multiple access collision avoidance for wireless local area network( MACAW) protocols in underwater acoustic channel environment are evaluated. The different performance of protocols in underwater environment is compared in the simulation.

Utility-optimization dynamic subcarrier allocation algorithm for SC-FDMA systems

Two utility-optimization dynamic subcarrier allocation(DSA) algorithms are designed for single carrier frequency division multiple access system(SC-FDMA).The two proposed algorithms aim to support diverse transmission capacity requirements in wireless networks,which consider both the channel state information(CSI) and the capacity requirements of each user by setting appropriate utility functions.Simulation results show that with considerable lower computational complexity,the first utility-optimization algorithm can meet the system capacity requirements of each user effectively.However,the rate-sum capacity performance is poor.Furthermore,the second proposed utility-optimization algorithm can contribute a better trade-off between system rate-sum capacity requirement and the capacity requirements of each user by introducing the signal to noise ratio(SNR) information to the utility function based on the first utility-optimization algorithm,which can improve the user requirements processing capability as well as achieve a better sum-rate capacity.

Energy efficiency aware dynamic rate and power adaptation in carrier sensing based WLANs under Rayleigh fading and shadowing

We consider the problem of energy efficiency aware dynamic adaptation of data transmission rate and transmission power of the users in carrier sensing based Wireless Local Area Networks(WLANs)in the presence of path loss,Rayleigh fading and log-normal shadowing.For a data packet transmission,we formulate an optimization problem,solve the problem,and propose a rate and transmission power adaptation scheme with a restriction methodology of data packet transmission for achieving the optimal energy efficiency.In the restriction methodology of data packet transmission,a user does not transmit a data packet if the instantaneous channel gain of the user is lower than a threshold.To evaluate the performance of the proposed scheme,we develop analytical models for computing the throughput and energy efficiency of WLANs under the proposed scheme considering a saturation traffic condition.We then validate the analytical models via simulation.We find that the proposed scheme provides better throughput and energy efficiency with acceptable throughput fairness if the restriction methodology of data packet transmission is included.By means of the analytical models and simulations,we demonstrate that the proposed scheme provides significantly higher throughput,energy efficiency and fairness index than a traditional non-adaptive scheme and an existing most relevant adaptive scheme.Throughput and energy efficiency gains obtained by the proposed scheme with respect to the existing adapting scheme are about 75%and 103%,respectively,for a fairness index of 0.8.We also study the effect of various system parameters on throughput and energy efficiency and provide various engineering insights.

Phase separations in graded-indium content InGaN/GaN multiple quantum wells and its function to high quantum efficiency

Phase separations have been studied for graded-indium content In_xGa_（1-x）N/GaN multiple quantum wells（MQWs） with different indium contents by means of photoluminescence（PL）,cathodeluminescence（CL） and time-resolved PL（TRPL） techniques.Besides the main emission peaks,all samples show another 2 peaks at the high and low energy parts of the main peaks in PL when excited at 10 K.CL images show a clear contrast for 3 samples,which indicates an increasing phase separation with increasing indium content.TRPL spectra at 15 K of the main emissions show an increasing delay of rising time with indium content,which means a carrier transferring from low indium content structures to high indium content structures.

Analysis of Multi-Channel and Slotted Random Multi-Access Protocol with Two-Dimensional Probability for Ad Hoc Network

A higher quality of service （QoS） is provided for ad hoc networks through a multi-channel and slotted random multi-access （MSRM） protocol with two-dimensional probability. For this protocol, the system time is slotted into a time slot with high channel utilization realized by the choice of two parameters p1 and p2, and the channel load equilibrium. The protocol analyzes the throughput of the MSRM protocol for a load equilibrium state and the throughput based on priority. Simulations agree with the theoretical analysis. The simulations also show that the slotted-time system is better than the continuous-time system.

Carrier multiplication in germanium nanocrystals

Carrier multiplication is demonstrated in a solid-state dispersion of germanium nanocrystals in a silicon–dioxide matrix.This is performed by comparing ultrafast photo-induced absorption transients at different pump photon energies below and above the threshold energy for this process.The average germanium nanocrystal size is approximately 5–6 nm,as inferred from photoluminescence and Raman spectra.A carrier multiplication efficiency of approximately 190%is measured for photo-excitation at 2.8 times the optical bandgap of germanium nanocrystals,deduced from their photoluminescence spectra.

Towards Qo E-based resource allocation schemes in SC-FDMA systems

One of the critical techniques of quality of experience(Qo E) provisioning in single carrier frequency division multiple access(SC-FDMA) systems is related to implementation of efficient radio resource allocation(RRA) schemes. So far as we know, Qo E-based RRA in SC-FDMA systems is still an open issue. Therefore, in this paper, five different Qo E-based RRA schemes in SC-FDMA system are investigated based on different criteria. The first three Qo E criteria applied in other wireless systems are maximizing average mean opinion score(MOS) over all users(Max-MOS), maximizing the minimum MOS among all users(Max Min-MOS) and maximizing the minimum MOS with dropping some users requiring the highest amount of resources(Max Min-Min MOSX.Y-MOS). Then, the improved Max Min-Min MOSX.Y-MOS criterion focuses on changing user dropping principle to promote the probability of more users obtaining satisfied experience. By defining MOS higher than threshold as the effective MOS, maximizing average effective MOS over all users(Max-EMOS) is proposed as a totally new criterion which gives quantitative measurement of how many users to what extent obtaining satisfied experience. Mathematical expressions and solutions of these schemes are also presented in this paper. In addition, performance comparisons are made in simulations to further display their respective advantages and disadvantages.