Active database systems support mechanisms that enable them to respond automatically to events that are taking place either inside or outside the database system itself. Considerable effort has been directed towards improving understanding of such systems in recent years, and many different proposals have been made and applications suggested. This high level of activity has not yielded a single agreed-upon standard approach to the integration of active functionality with conventional database systems, but has led to improved understanding of active behavior description languages, execution models, and architectures. This survey presents the fundamental characteristics of active database systems, describes a collection of representative systems within a common framework, considers the consequences for implementations of certain design decisions, and discusses tools for developing active applications.

In this paper, the application of orthogonal frequency division multiplexing (OFDM) over an indoor wireless optical channel is investigated. Previous studies have demonstrated that it can be characterised as almost stationary but with a severe penalty induced by multipath propagation. It is also highly dependent on the spatial distribution of emitters and receivers. OFDM systems have been chosen owing to their capability of supporting high data rates without channel equalisation. They also mitigate the effect of quality fluctuations of the channel induced when the spatial distribution of emitters and receivers varies. We present several results about system performance. Finally, an adaptive OFDM system is proposed to increase data throughput. The obtained results show that a significant throughput is attained over noisy wireless optical channels.