Abstract
In this paper we present a comprehensive analysis and a performance assessment on the transmission of digitised RF signals over optical fibre (DRoF). Specifically, a study of the impact of the ADC/DAC quantization, jitter noise and the signal attenuation caused by the sub-sampling technique and DAC frequency roll-off are addressed by means of simulation, considering the transmission of RF signals conveying QAM symbols. Additionally, an experimental evaluation of DRoF links using vertical-cavity surface-emitting lasers (VCSEL) for different optical fibre attenuation levels is also conducted. Finally, a new paradigm of DRoF systems based on well-known sigma-delta modulators (SDM) is also presented. Results show that it is possible to digitally transmit signals through a digital optical-based network and to distribute them wirelessly at the receiver side without the need for local oscillators (LO) and/or frequency up-converters. Results also show that the new sigma-delta-over-fibre concept performs similarly to conventional DRoF schemes, whilst being more competitive for either upgrading installed systems as well as for new deployments.

Abstract
In this paper, we propose a new channel estimation (CE) structure that aims to minimize the mean square error (MMSE) of orthogonal frequency-division multiplexing signals that are corrupted by fading, nonlinear, and additive white Gaussian noise channels. We demonstrate that it exhibits a simple structure consisting of a set of correlators matched to every frequency component in the nonlinearly distorted signal, followed by a linear transformation. An extension suitable for frequency shifted signals is also discussed. As case study, the potential of such CE in uplink radio-over-fiber (RoF) systems conveying WiFi signals is analyzed. Through simulations, we demonstrate the usage of the one-tap and iterative maximum likelihood (ML)-algorithm-based equalizers, when used jointly with the proposed MMSE, zero forcing, or perfect CEs. Results show that the performance of the MMSE CE approximates the one of the perfect CE. In addition, when used in conjunction with the ML algorithm, it enables relaxation of linear requirements of components in RoF by passing the need for transmitter predistortion techniques that would otherwise significantly increase wireless devices complexity.

Abstract

Abstract
In this work the performance assessment of a digitised radio-over-fibre (DRoF) system is presented considering the transmission of orthogonal frequency division multiplexing (OFDM) signals. Specifically, the impact of the high peak-to-average-ratio (PAPR) in the ADC/DAC quantization noise is addressed by means of simulation results. In order to minimize the impact of the digitisation process, we present a study of the improvement of the system performance when commonly used pre-distortion techniques suitable for PAPR reduction are considered. Additionally, the performance of the selected pre-distortion techniques is assessed experimentally, in terms of the modulation error ratio (MER) in an OFDM-DRoF link transmission, considering the impact of optical attenuation. It is observed that the transmission of OFDM signals in DRoF is severely affected by the inherent high PAPR levels and that, by using pre-distortion techniques with ADC/DAC with 7 bits, it is possible to increase the modulation error ratio (MER) by 15 dB or to increase the optical fibre by 6.5 km (8.5% increase) without the need of additional complexity at the OFDM receiver.

Abstract
The Web is rapidly becoming the prime medium for human socialization. As it evolves towards an information-centric operation, it records everything and forgets nothing, assuming that every online resource disclosed by people (photos, posts, multimedia files, etc.) is permanently valid and is to be stored forever. However, throughout their lives, people tend to change, both in their habits as well as in their views and opinions. In many situations, as the years go by, information released loses relevance or people may decide they no longer want others to access information they have previously published. The work presented in this paper strives for a new information persistence paradigm, whereby the enforcement of "digital forgetting" is implemented over an information-centric model for the Internet. The defined solution enables the definitive elimination of digital objects, either on-demand or on a pre-scheduled basis, and, hence, their "forgetting." The solution, conceived within the framework of the European project CONVERGENCE, is based on the employment of metadata descriptions about resources, which unambiguously identify their rightful owners. This additional data is efficiently bound to the resource through the use of an extended version of the MPEG-21 Digital Item specification, and its prescriptions are enforced by CONVERGENCE's distributed provisions.

Abstract