https://www.um.edu.mt/library/oar/handle/123456789/14674 2026-04-15T06:03:50Z https://www.um.edu.mt/library/oar/handle/123456789/123340 Title: Open source activities social network Abstract: Through this Final Year Project, we wanted to study whether using a software engineering approach and intelligent techniques, we could implement a system that would help committees and organising teams in their daily running. Through the various requirement gathering techniques used and the background literature review that were carried out, we were able to come out and implement a system which aims to specifically do so. In the evaluation of the system we see that by the use of the Agile software development life cycle we were able to build prototypes which then made up a very robust system, with very few little bugs being reported during the beta testing. We were also able to implement the system in a way to be as easy to use as possible, which was depicted by our users themselves. By the use of the recommendation engine, we were able to give personalised recommendations to the users in order to tempt them to contribute to other similar events. During the focus group interview that was carried out, we were able to conclude that it served as a great addition to our social networking site. Even more, having gathered the users Facebook likes, the engine was able to compute better recommendations. Last but not least, in order to help committees we had to evaluate the three main features that were introduced in CALBUD: the Document Management System, Meeting Management System and the Group Chat. All of these were rated with a high rating from the beta testers, and during the focus group interview we were able to depict that by the use of the meeting management system and the document management system, committees will lose less time during the meetings themselves. This is since the system allows them to prepare and approve the agenda and minutes collaboratively and also allows them to annotate on media files. Description: B.SC.(HONS)ICT 2013-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/101196 Title: Multicast multimedia transmission over wireless local area networks Abstract: Multicast over IEEE 802.1 la/b/g/n Wireless Local Area Networks (WLANs) is an efficient means of transmission exploiting the broadcast nature of the wireless medium. Moreover the use of the unlicensed 2.4GHz or the 5GHz bands does not incur licensing costs. But IEEE 802.lla/b/g/n Access Points (APs) transmit multicast unreliably, because the receivers do not transmit feedback regarding packet reception. Hence, multicast over IEEE 802.lla/b/g/n WLANs suffers from a high Packet Error Rate (PER) which is inappropriate for multimedia transmission. In 2012, the IEEE 802.llaa standard added reliability to multicast using Directed Multicast Service, Groupcast with Retries (GCR) Unsolicited Retry and GCR Block Acknowledgement. However, packet repetition does not result in an optimal code rate. The work in this thesis first verified, using empirical and semi-analytical analyses, that two antennas of an IEEE 802.1 ln AP can be spatially distributed to mitigate the PER experienced. A main contribution was the proposal of a distributed antennas system (DAS), consisting of seven antennas, which was shown to multicast video over IEEE 802.1 ln WLANs with a Peak Signal-to-Noise Ratio (PSNR) of at least 36dB, with the same power but better code rate than other infrastructures. Even packets are multicast over one set of four transmit antennas with an antenna placed in the centre of the coverage area and three transmit antennas placed equidistantly at the periphery. The centre antenna and three other transmit antennas, also placed equidistantly at the periphery, are then used to multicast odd packets. The proposed DAS can also use antenna switching so that unicast transmission uses a centralized antennas system. The proposed DAS is shown to scale up well using a multi-cell approach. It was shown that packet repetition at the application layer, retransmitting each packet proactively once (code rate 0.5), does not result in the entire multicast group receiving good Quality of Service (QoS) with a legacy infrastructure or a DAS using only one set of four transmit antennas, due to burst erasures on the channel. The proposed seven-antenna DAS does guarantee the required QoS to the entire multicast group, on the other hand. Hence, schemes such as IEEE 802.11 aa GCR Unsolicited Retry should be deployed with the proposed DAS. The best code rate is achieved with Block Erasure Coding (BEC) on the proposed DAS. However, Network Coding, achieved by XORing packets to create parity packets, achieves a higher code rate than packet repetition and a lower delay than BEC, and results in good QoS with a PHY data rate of 58.5Mbps. Another original contribution is the proposal and study of two new MAC layer protocols. The first protocol uses a Binary Search guided by feedback from the receivers to determine the necessary number of R-S encoded parity packets. The second protocol combines Network Coding and packet repetition with feedback from the receivers, resulting in smaller channel occupancy than reactive packet repetition. Although this second protocol results in a larger maximum delay than reactive packet repetition, it is still appropriate for live-video streaming since the maximum delay is less than 8 ms. Finally, an infrastructure using distributed IEEE 802.1 ln APs is shown to perform better than the legacy infrastructure, at a code rate of 1/3. The advantage of using distributed APs is that IEEE 802.1 ln MCSs employing Spatial Division Multiplexing can be used for multicasting. Description: PH.D. 2013-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/101141 Title: Fast automatic beam-based alignment of the LHC collimator jaws Abstract: The CERN Large Hadron Collider (LHC) in Geneva, Switzerland is the largest and most powerful particle accelerator ever built. With a circumference of 27 km, it is designed to collide particles in two counter-rotating beams, at a centre-of-mass energy of 14 TeV to explore the fundamental forces and constituents of matter. Due to its potentially destructive high energy particle beams, the LHC is equipped with several machine protection systems. The LHC collimation system is tasked with scattering and absorbing beam halo particles before they can quench the superconducting magnets. The 108 collimators also protect the machine from damage in the event of very fast beam losses, and shields sensitive devices in the tunnel from radiation over years of operation. Each collimator is made up of two blocks or 'jaws' of carbon, tungsten or copper material. The collimator jaws need be placed symmetrically on either side of the beam trajectory, to clean halo particles with maximum efficiency. The beam orbit and beam size need to be determined for each collimator, to be able to position the jaws Within a certain number of standard deviations (beam o-) from the beam centre. Beam-based alignment is used to determine these values at every collimator location. In the alignment procedure, each jawv is moved separately towards the beam trajectory, in 5 inn steps, until a spike appears in the signal of a Beam Loss Monitoring (BLM) detector positioned a couple of metres downstream of the collimator. A balance is required between scraping enough beam to obtain a signal, avoiding automatically triggered beam extractions (or dumps) in the event. of high beam losses, and completing the alignment in the shortest time possible to allow the LHC to produce maximum luminosity. In the 2010 LHC run, almost 30 hours were required for an alignment of all collimators, and 8 beam dumps were caused due to operator mistakes. A phased development, commissioning and usage of various algorithms in the 2011-2012 LHC runs allowed the alignment time to decrease to limit over 4 hours, with no more beam dumps. The algorithms range from automatic selection of BLM thresholds during the alignment, to BLM-based feedback loops and pattern recognition of the BLM signal spikes. The BLM-based feedback loop was also successfully used by the ALFA and TOTEM particle physics experiments in Roman Pot alignment campaigns. A Roman Pot is a detector that intercepts slightly deflected particles from head-on collisions to measure the total collision rate (cross-section). An alignment simulator was developed in MATLAB based on an empirical model of the BLM detector signal steady-state and crosstalk, as well as a beam diffusion model which allows the prediction of the characteristic BLM detector signal spike and decay. The simulator is targeted at validating possible future alignment algorithms which would otherwise require dedicated beam tests. A new collimator design for future LHC operation envisages Beam Position Monitor (BPM) pick-up buttons embedded inside the jaws. The BPM's will provide an accurate and continuous measurement of the beam centres without requiring BLM-based alignment. One quarter of the LHC collimators (tertiary collimators and IR6 secondary collimators) will be replaced with the new design, as foreseen since several years. Hence, an algorithm to automatically position the jaws around the beam centre at a large jaw gap was developed and tested with a prototype mock-up collimator installed in the Super Proton Synchrotron (SPS). Alignment times of approximately 20 s were reached. The work described in this dissertation was adopted by CERN for the first LHC running period (2008 - 2013). It will continue to be used in future operation post2015 after a two-year shutdown, in which the machine will be upgraded to be able to operate at the design parameters. Description: PhD 2013-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/100910 Title: Source representation for improved channel code performance in Wyner-Ziv video coding Abstract: The Wyner-Ziv video coding paradigm is a new coding paradigm which exploits most of the source correlation at the decoder. This differs from the traditional predictive video coding schemes, where the source correlations are exploited solely at the encoder. Hence, the new paradigm can enable the implementation of low complexity encoders suitable for various applications such as endoscopy capsules and low-power surveillance systems. Slepian-Wolf (SW) and Wyner-Ziv (WZ) theorems prove that when the complexities, of exploring the source statistics, are shifted from the encoder to the decoder, the coding efficiency should not be affect. Hence, under certain conditions, the coding performance of WZ video coding schemes can theoretically be made arbitrarily close to that of conventional schemes where the sources are jointly encoded and decoded. However, the Rate-Distortion (R-D) performance of practical Wyner-Ziv video coding architectures are still far from the best performance attained with predictive video coding architectures like the H264/AVC or High Efficiency Video Coding (HEVC) video coding scheme. This Thesis investigates several methods to improve the performance of Slepian-Wolf coding, in terms of coding efficiency and reduced decoding delays. It is noticed that the traditional Slepian-Wolf coding approaches encode the bits within the same bit-plane level randomly using Low-Density Parity-Check Accumulate (LDPCA) codes and this leads to a sub-optimal performance. The reliability of the bits can be predicted and used to ensure that the bit nodes receiving low reliability bit-predictions are given better protection. A novel LDPCA code construction, targeted to consider the coding problem at hand, is thus proposed. Furthermore, this work also analyses the performance of the traditional LDPCA codes at different entropy points and studies the best way to distribute the correlation noise amongst bit-planes to improve coding efficiency. This is achieved by accumulating the most unreliable bits within the first decoded bit-planes and correcting the remaining bit-planes, having few bit-errors, using 8-bit or 16-bit Cyclic Redundancy Check (CRC) codes. The careful arrangement of discrepancies amongst bit-planes is used together with the arrangement of bits within each bit-plane and the new LDPCA codes, to obtain performance gains of up to 23 % during Slepian-Wolf coding. In context of Slepian-Wolf coding, the Thesis also addresses a comprehensive analysis of the mismatch present within regions of low motion. This is used to develop a scheme where the quantisation module alters between the floor and the round operator, at different pixel or coefficient locations. The operator which is more likely to avoid the Slepian-Wolf codec from correcting mismatch caused by small variation in light intensity is then chosen to improve R-D performance by up to 0.52 dB. Finally, the long decoding times required for Slepian-Wolf decoding are reduced by considering a new indexing scheme and histogram equalisation technique. For parallel WZ video coding architectures, these techniques ensure that the Slepian-Wolf decoders running on different cores of a multi-core processor can finish decoding at the same time, aiding parallel decoding and reducing decoding times by up to 32 %, with minimal affect on the R-D performance. The obtained reduction in rates and decoding delays helps bridge the gap in performance compared to the traditional video coding systems and pave the pathway for applications based on WZ video coding paradigm. Description: PH.D. 2013-01-01T00:00:00Z