This paper describes an approach, based on a system, called Threads, which is already in commercial use, to automatically synchronise sets of digital messages (e.g. e-mail and digitised phone calls) with particular projects, in which context the messages were created. Based on the authors' experience that it is generally easier to browse projects of interest than to search masses of messages, authorised users can extract information from far more messages, far quicker than they might otherwise do by trying to guess suitable search terms. The novelty described in this paper is a method for organising such messages automatically and intelligently according to statistics relating to less common words (called "keywords") which they contain. Initial experiments using this method are described, using both e-mail data from the parent company, and on the publicly available ENRON dataset of e-mails and phone calls. These preliminary results are interesting, but suggest the method as currently implemented is only suitable for semi-automatic classification, but not yet for fully automated allocations to projects.

Delay matters in future wireless communication. An appropriate limit for rates achievable under delay constraints is the delay limited capacity (DLC). In this work, the DLC of OFDM systems is investigated. Despite its complicated correlation structure the OFDM DLC is fully characterized for low and high SNR. It is shown that (under weak assumptions) the OFDM DLC is almost independent of the fading distribution in the low SNR region but strongly depends on the delay spread thereby achieving a capacity gain over AWGN capacity. In the high SNR region the roles are exchanged. Here, the impact of delay spread is negligible while the impact of the fading distribution becomes dominant. The relevant quantities and their asymptotic behaviour are derived without employing simplifying assumptions on the OFDM correlation structure. Using a general convergence framework the analysis further shows that if the delay spread becomes large even the predominant impact of the fading distribution vanishes and DLC capacity loss compared to AWGN capacity approaches 0.58[nats/s/Hz]. The convergence speed, the loss due to non-uniform power delay profile, and the relation to ergodic capacity is also analyzed and underlined with simulations and application examples. The main conclusion here is that OFDM fully takes advantage of the degrees of freedom of the underlying fading channel in terms of delay spread and, regardless of the fading distribution, delay sensitive capacity measures such as the DLC converge to the ergodic capacity. Finally, since universal bounds are obtained which apply to any fading distribution the results can also be used for other classes of parallel channels extending the range of applicability.

In this paper we describe an experiment, in which we examined older and younger adults when interacting with a simulated PDA (personal digital assistant). Independent variables were users' age (young vs. older) and device interface (hyperlink vs. no hyperlink). Dependent variables were the effectiveness and efficiency of menu navigation. To understand how user characteristics influence performance, spatial ability, verbal memory, computer expertise and technical self-confidence were determined. Technology experienced young and older adults (benchmark testing) took part. They had to solve four tasks either with hyperlink interface or without hyperlinks in the interface. The method to collect, to automatically analyze and to structure the data according to interaction sequences and presumed user intentions is a novel approach supported by the open source software tool Clever [12]. The tool is briefly described; more details can be found in [23]. Results revealed that hyperlink interfaces showed overall higher effectiveness. However, the impact of hyperlinks for efficiency was age-related. Younger adults strongly benefit from having hyperlinks. The contrary was the case for older adults, who showed higher menu disorientation when using hyperlinks.

The main task in OFDM downlink scheduling design is to maximize some kind of utility considering user specific as well as system induced constraints. A typical example is the weighted sum rate maximization under a sum power constraint and possibly additional rate requirements. A major challenge is to incorporate the fact that in real systems the number of modulation depths and coding schemes is limited due to technical conditions. Thus, the scheduling problem results in a discrete optimization problem with non-differentiable non-convex objective. In this paper a new approach is presented that can deal with this kind of constraints in a general way. By transforming the discrete problem in a suitable weight matching problem an efficient algorithm which is easy to implement is devised. The performance of this algorithm is evaluated by simulations with parameters obtained from the recently proposed OFDM-HSDPA concept [1].

We consider the downlink of a multi-antenna system where the transmitter performs nonlinear dirty-paper coding (DPC) and each receiver is equipped with a single antenna. Assuming that the base station must support a certain quality of service (QoS) for each user, we analyze the minimum power transmit strategy consisting of filters, power allocation and precoding order. We derive necessary and sufficient conditions for the optimality of pure strategies, i.e. strategies achieving the QoS without time-sharing. Based on these conditions, we present a very simple algorithm yielding a suboptimal pure strategy showing excellent performance. This is of considerable interest especially for real systems, where time-sharing is hard to implement.

A system consisting of$K$parallel Gaussian broadcast channels over which a transmitter communicates with$M$receivers is considered. Assuming perfect channel state information, we study the problem of maximizing a weighted sum of rates under a sum power constraint with additional receiver-specific rate constraints. This problem formulation occurs frequently, e.g., in the context of scheduling for cellular downlink systems. Two algorithms are proposed which of one is based on a modified water-filling procedure. This algorithm is also suited to solve the minimum sum power problem.

In this paper, we solve the weighted sum rate problem for an orthogonal frequency division multiplexing (OFDM) broadcast channel (BC) under a sum power constraint, if minimum rates have to be guaranteed in each fading state and perfect channel state information (CSI) is assumed at the base station and the mobiles. The problem is subdivided into two problems. First, we tackle the problem of feasibility, which occurs since the system is power limited and not all required rates might be supportable. Subsequently, the optimal resource allocation in case of feasibility is derived. Moreover, the optimal decoding order, which is not determined by the problem formulation itself, is obtained as the ordering of the Lagrangian factors of the main problem. Finally, we show that the problem is closely related to the weighted rate sum maximization and sum power minimization and all three can be interpreted in a unifying framework embedded in a higher dimensionality

In this work, we calculate the delay-limited capacity (DLC) region of an OFDM (orthogonal frequency division multiplexing) broadcast channel. Furthermore, we analyze the impact of system parameters. The main results are that under weak assumptions, the single user DLC is almost independent of the distribution of the path attenuations in the low SNR region but depends strongly on the delay spread. In the high SNR region the roles are exchanged. Here, the impact of delay spread is negligible and the impact of the distribution is dominant. Finally, we provide a result for the multiuser case.

In this paper, we address the problem of optimal power allocation for multiple input multiple output (MIMO) orthogonal frequency division multiplexing (OFDM) broadcast channels with a sum power constraint when transmitter and receiver have perfect channel state information (CSI). We derive optimality conditions for power allocation and present an efficient algorithm achieving the optimal power allocation. Furthermore, we obtain necessary and sufficient conditions for the optimality of frequency division multiple access (FDMA) transmission and single user transmission. It turns out that the assignment of multiple users to one subcarrier is optimal in general. We propose a low complexity algorithm using a pure FDMA-scheme without multiple assignment of users and characterize the achievable rate region in comparison to the complete capacity region.