RLS-based initialization for per-tone equalizers in DMT receivers

Per-tone equalization has recently been proposed as an alternative receiver structure for discrete multitone-based systems improving upon the well-known structure based on time-domain equalization. Fast initialization of all the equalizer coefficients has been identified as an open problem. In this letter, a recursive initialization scheme based on recursive least squares with inverse updating is presented for the per-tone equalizers. Simulation results show convergence with an acceptably small number of training symbols. Complexity calculations are made for per-tone equalization and for the case where tones are grouped. It is demonstrated with an example that in the latter case, initialization complexity becomes sufficiently low and comparable to complexity during data transmission.     

Exact algorithms for a generalization of the order acceptance and scheduling problem in a single-machine environment

This paper studies a generalization of the order acceptance and scheduling problem in a single-machine environment where a pool consisting of firm planned orders as well as potential orders is available from which an over-demanded company can select. The capacity available for processing the accepted orders is limited and each order is characterized by a known processing time, delivery date, revenue and a weight representing a penalty per unit-time delay beyond the delivery date. We prove that the existence of a constant-factor approximation algorithm for this problem is unlikely. We propose two linear formulations that are solved using an IP solver and we devise two exact branch-and-bound procedures able to solve instances with up to 50 jobs within reasonable CPU times. We compare the efficiency and quality of the results obtained using the different solution approaches.     

Multilayer emulsions as delivery systems for controlled release of volatile compounds using pH and salt triggers

Multilayer oil-in-water (M-O/W) emulsions were compared to primary oil-in-water (P-O/W) emulsions as carriers for volatile organic compounds (VOCs) under various environmental conditions (pH and salt). The M-O/W emulsion consisted of soy oil coated with β-lactoglobulin (βLG) and pectin layers. The release of VOCs with different physiochemical properties from aqueous solutions and emulsion systems was measured using static and dynamic headspace methods. The partition coefficients (K) calculated by the phase ratio variation (PRV) method, showed different volatile release profiles between the emulsion types. An increase in VOC release was found for the unstable P-O/W emulsion at pH 5, whereas M-O/W emulsions were stable at the same pH and retained the hydrophobic VOCs. Hydrophobic interactions and hydrogen bonds with the secondary dense layer of pectin may be responsible for the improved retention. Increasing pH and ionic strength acts as a VOC release trigger to detach the pectin from the interface. The release rates from initial dynamic curves support the results under equilibrium conditions. The results of this study demonstrate the capability of using M-O/W emulsions for controlled release of VOCs, as well as an alternative system to create stable emulsions with similar VOC release profiles.     

Metal-Organic Frameworks as Selective or Chiral Oxidation Catalysts

Since the discovery of Metal Organic Frameworks (MOFs) in the early 1990s, the amount of new structures has grown exponentially. A MOF typically consists of inorganic nodes that are connected by organic linkers to form crystalline, highly porous structures. MOFs have attracted a lot of attention lately, as the versatile design of such materials holds promises of interesting applications in various fields. In this review, we will focus on the use of MOFs as heterogeneous oxidation catalysts. MOFs are very promising candidates to replace homogeneous catalysts by sustainable and stable heterogeneous catalysts.

The catalytic active function can be either the active metal sites of the MOF itself or can be introduced as an extra functionality in the linker, a dopant or a “ship-in-a-bottle” complex. As the pore size, pore shape, and functionality of MOFs can be designed in numerous ways, shape selectivity, and even chiral selectivity can be created. In this article, we will present an overview on the state of the art of the use of MOFs as a heterogeneous catalyst in liquid phase oxidation reactions.     

New competitive results for the stochastic resource-constrained project scheduling problem: exploring the benefits of pre-processing

We study the resource-constrained project scheduling problem with stochastic activity durations. We introduce a new class of scheduling policies for solving this problem, which make a number of a-priori sequencing decisions in a pre-processing phase while the remaining decisions are made dynamically during project execution. The pre-processing decisions entail the addition of extra precedence constraints to the scheduling instance, hereby resolving some potential resource conflicts. We obtain new competitive results for expected-makespan minimization on representative datasets, which are significantly better than those obtained by the existing algorithms when the variability in the activity durations is medium to high.     

R&D project scheduling when activities may fail

An R&D project typically consists of several stages. Due to technological risks, the project may have to be terminated before completion, each stage having a specific likelihood of success. In the project planning and scheduling literature, this technological uncertainty has typically been ignored and project plans are developed only for scenarios in which the project succeeds. In this paper we examine how to schedule projects in order to maximize their expected net present value when the project activities have a probability of failure and when an activity's failure leads to overall project termination. We formulate the problem, show that it is NP-hard, develop a branch-and-bound algorithm that allows us to obtain optimal solutions and provide extensive computational results. In the process, we establish a complexity result for an open problem in single-machine scheduling, namely for the discounted weighted-completion-time objective with general precedence constraints.     

Per-tone equalization for MIMO OFDM systems

This paper focuses on multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems with channel order larger than the cyclic prefix (CP) length. Writing the demodulating fast Fourier transform (FFT) as a sliding FFT followed by a downsampling operation, we show in this paper that by swapping the filtering operations of the MIMO channel and the sliding FFT, the data model for the temporally smoothened received signal of each individual tone of the MIMO OFDM system is very similar to the data model for the temporally smoothened received signal of a MIMO single-carrier (SC) system. As a result, to recover the data symbol vectors, the conventional equalization approach for MIMO SC systems can be applied to each individual tone of the MIMO OFDM system. This so-called per-tone equalization (PTEQ) approach for MIMO OFDM systems is an attractive alternative to the recently developed time-domain equalization (TEQ) approach for MIMO OFDM systems. In the second part of this paper, we focus on direct per-tone equalizer design and adapt an existing semi-blind equalizer design method for space-time block coding (STBC) SC systems to the corresponding semi-blind per-tone equalizer design method for STBC OFDM systems.     

Equalization for OFDM over doubly selective channels

In this paper, we propose a time-domain as well as a frequency-domain per-tone equalization for orthogonal frequency-division multiplexing (OFDM) over doubly selective channels. We consider the most general case, where the channel delay spread is larger than the cyclic prefix (CP), which results in interblock interference (IBI). IBI in conjunction with the Doppler effect destroys the orthogonality between subcarriers and, hence, results in severe intercarrier interference (ICI). In this paper, we propose a time-varying finite-impulse-response (TV-FIR) time-domain equalizer (TEQ) to restore the orthogonality between subcarriers, and hence to eliminate ICI/IBI. Due to the fact that the TEQ optimizes the performance over all subcarriers in a joint fashion, it has a poor performance. An optimal frequency-domain per-tone equalizer (PTEQ) is then obtained by transferring the TEQ operation to the frequency domain. Through computer simulations, we demonstrate the performance of the proposed equalization techniques.     

Blind and semi-blind equalization for generalized space-time block codes

This paper presents a general framework for space-time codes (STCs) that encompasses a number of previously proposed STC schemes as special cases. The STCs considered are block codes that employ arbitrary redundant linear precoding of a given data sequence together with embedded training symbols, if any. The redundancy introduced by the linear precoding imposes structure on the received data that under certain conditions can be exploited for blind or semi-blind estimation of the transmitted sequence, a linear receiver that recovers the sequence, or both simultaneously. Algorithms based on this observation are developed for the single-user flat-fading case and then extended to handle multiple users, frequency-selective fading, as well as situations where the channel is rank deficient, or there are fewer receive than transmit antennas.     

Constant modulus and reduced PAPR block differential encoding for frequency-selective channels

Frequency-selective channels can be converted to a set of flat-fading subchannels by employing orthogonal frequency-division multiplexing (OFDM). Conventional differential encoding on each subchannel, however, suffers from loss of multipath diversity, and a very high peak-to-average power ratio (PAPR), which causes undesirable nonlinear effects. To mitigate these effects, we design a block differential encoding scheme over the subchannels that preserves multipath diversity, and in addition, results in constant modulus transmitted symbols. This property is shown to ensure that the PAPR of the continuous-time transmitted waveform is reduced by a large factor. The maximum-likelihood decoder for the proposed scheme, conditioned on the current and previous received block, is shown to have linear complexity in the number of subcarriers. The constant modulus scheme will yield good bit-error rate performance with full rate only if short blocks are used. However, one may mitigate this problem by relaxing the constant modulus requirement. We show that in a practical OFDM system, we can group the subcarriers into shorter subblocks in a certain manner, and apply the constant modulus technique to each subblock. Thus, we improve diversity at a very low decoder complexity, and at the same time, we introduce an upper bound on the discrete-time PAPR, which, in turn, may lead to appreciable reduction in continuous-time PAPR, depending on the system parameters. Finally, in situations where we can sacrifice rate, additional complex field coding may be used to exploit the multipath diversity provided by channels longer than those the simple scheme can handle.