Space-Time Coding and Space-Time Channel Modelling for Wireless Communications

In this thesis we investigate the effects of the physical constraints such as antenna aperture size, antenna geometry and non-isotropic scattering distribution parameters (angle of arrival/departure and angular spread) on the performance of coherent and non-coherent space-time coded wireless communication systems. First, we derive analytical expressions for the exact pairwise error probability (PEP) and PEP upper-bound of coherent and non-coherent space-time coded systems operating over spatially correlated fading channels using a moment-generating function-based approach. These analytical expressions account for antenna spacing, antenna geometries and scattering distribution models. Using these new PEP expressions, the degree of the effect of antenna spacing, antenna geometry and angular spread is quantified on the diversity advantage (robustness) given by a space-time code. It is shown that the number of antennas that can be employed in a fixed antenna aperture without diminishing the diversity advantage of a space-time code is determined by the size of the antenna aperture, antenna geometry and the richness of the scattering environment. ¶ In realistic channel environments the performance of space-time coded multiple-input multiple output (MIMO) systems is significantly reduced due to non-ideal antenna placement and non-isotropic scattering. In this thesis, by exploiting the spatial dimension of a MIMO channel we introduce the novel use of linear spatial precoding (or power-loading) based on fixed and known parameters of MIMO channels to ameliorate the effects of non-ideal antenna placement on the performance of coherent and non-coherent space-time codes. ..

Design Guidelines for Training-based MIMO Systems with Feedback

In this paper, we study the optimal training and data transmission strategies for block fading multiple-input multiple-output (MIMO) systems with feedback. We consider both the channel gain feedback (CGF) system and the channel covariance feedback (CCF) system. Using an accurate capacity lower bound as a figure of merit, we investigate the optimization problems on the temporal power allocation to training and data transmission as well as the training length. For CGF systems without feedback delay, we prove that the optimal solutions coincide with those for non-feedback systems. Moreover, we show that these solutions stay nearly optimal even in the presence of feedback delay. This finding is important for practical MIMO training design. For CCF systems, the optimal training length can be less than the number of transmit antennas, which is verified through numerical analysis. Taking this fact into account, we propose a simple yet near optimal transmission strategy for CCF systems, and derive the optimal temporal power allocation over pilot and data transmission.Comment: Submitted to IEEE Trans. Signal Processin

MIMO Channel Correlation in General Scattering Environments

This paper presents an analytical model for the fading channel correlation in general scattering environments. In contrast to the existing correlation models, our new approach treats the scattering environment as non-separable and it is modeled using a bi-angular power distribution. The bi-angular power distribution is parameterized by the mean departure and arrival angles, angular spreads of the univariate angular power distributions at the transmitter and receiver apertures, and a third parameter, the covariance between transmit and receive angles which captures the statistical interdependency between angular power distributions at the transmitter and receiver apertures. When this third parameter is zero, this new model reduces to the well known "Kronecker" model. Using the proposed model, we show that Kronecker model is a good approximation to the actual channel when the scattering channel consists of a single scattering cluster. In the presence of multiple remote scattering clusters we show that Kronecker model over estimates the performance by artificially increasing the number of multipaths in the channel.Comment: Australian Communication Theory Workshop Proceedings 2006, Perth Western Australia. (accepted

Dependence of MIMO System Performance on the Joint Properties of Angular Power

In this paper, we use a novel MIMO channel model to characterize the dependence of ergodic capacity and diversity order on the joint statistics of the angular power density. The scattering environment of a MIMO channel is characterized by a double directional angular power distribution, describing the power transferred in each direction from transmitter aperture to receiver aperture. Angular power, which is typically separable Kronecker-modelled, is here generalized to include joint distribution properties using well-known bivariate probability density functions. We show that the joint properties of the power density, namely the shape and the orientation of power distribution contours, have significant impact on capacity and diversity of non-line-of-sight (NLOS) channels

Quadratic Variational Framework for Signal Design on the 2-Sphere

This paper introduces a quadratic variational framework for solving a broad class of signal design problems on the 2-sphere. The functional, to be extremized, combines energy concentration measures using a weighting function in the spatial domain, multiplicative weights in the spectral domain, and a total energy constraint. This leads to two formulations of the signal design problem on the 2-sphere, one a Fredholm integral equation in the spatial domain and the other an infinite matrix equation in the spectral domain. The framework is illustrated by deriving the key equations for the two classical spatio-spectral concentration problems on the 2-sphere, and for an isotropic filter design that maximizes the filtered energy. In addition, using the proposed framework, we formulate a joint 3-D beamforming application which achieves optimal directivity and spatial resolution simultaneously

A prospective cohort study to determine prognostic factors associated with outcomes in primary care attenders with unexplained physical symptoms

Background: Unexplained physical symptoms (UPS) that lack an organic explanation, even after appropriate investigation, are extremely common amongst UK primary care attenders but knowledge about their outcome is limited. / Aim: In a cohort of adult primary care attenders with UPS, this study aims to: 1) Investigate the outcome, in terms of the presence of UPS at six months follow-up and 2) Identify prognostic factors associated with somatic symptom severity, quality of life, anxiety, depression and health care use at six months follow-up. / Methods / Screening: Consecutive adults attending nine general practices completed a screening questionnaire to identify those with UPS. / Cohort study: Eligible participants completed the baseline questionnaire that enquired about somatic symptoms, quality of life, psychological well-being and past health and social history, and were followed-up after six months. / Results / Screening: Questionnaires were completed by 73% (2,826/3,896) of eligible attenders. Over two-thirds were female, median age was 42 years (IQR 30, 55) and median symptom severity score, based on the PHQ-15 was 7 (IQR 4, 11). Most (2,425/2,826 (86%)) had at least one UPS and around half (1,393/2,826 (49%)) had symptoms that were all unexplained (no explanation or diagnosis for any of their symptoms). Just under half (1,248/2,826, (44%)), had an explanation for their symptoms that included functional diagnoses (100/2,826 (4%)), psychological explanations (187/2,826 (7%)), or physical explanations or diagnoses (921/2,826 (33%)). / Cohort study: The cohort included 294 participants, were largely female (231/294 (79%)), with a median age of 44 years (IQR 32, 57)) and diverse ethnicity (43% white British). At baseline, the cohort had a high level of morbidity, with moderately severe somatic symptoms (11.5 SD 4.9). Most reported experiencing their symptoms for longer than a year. A third had clinically significant comorbid depression and anxiety. / Outcome: There was 245/294 (83%) followed-up at six months; mean PHQ-15 score was 10.5 (SD 5.3). Over a half reported unexplained symptoms (135/245 (55%)), just under half (103/245 (42%)) reported symptoms were still under investigation and only 26/245 (11%) reported that their symptoms had resolved. Options were not mutually exclusive and participants could choose more than one. The predictors of more severe somatic symptoms at follow-up were being female (B=1.31, 95% CI 0.12 to 2.50), higher somatic symptom severity (B=0.53, 95% CI 0.42 to 0.64), experience of childhood physical abuse (B=1.86 95% CI 0.27 to 3.45), perception of poor financial well-being (B=1.90, 95% CI 0.89 to 2.91) and lower physical functioning at baseline (B=-0.10, 95% CI -0.15 to -0.04). / Conclusion: Most people with UPS and high symptom severity are unlikely to improve over six months. Historical and current difficulties are associated with higher somatic symptom severity at follow-up. Future work should determine whether these findings are maintained over longer periods. The value of developing prognostic prediction models based on factors identified in this study should be explored

Relaying energy allocation in training-based amplify and forward relay communications

We consider relay-assisted communication in a training-based transmission scheme. Each transmission block consists of a training phase and a data transmission phase. The relay node employs the amplify-and-forward protocol during all transmissions. We focus on the relay signaling design and investigate the benefit of allowing for different relaying power during the training phase and the data transmission phase. Specifically, the relaying energy allocation between the two phases is optimized for maximizing the average received signal-to-noise ratio at the destination node. We study this optimization problem for both single-antenna relay and multi-antenna relay and derive a simple closed-form relaying energy allocation strategy that achieves near-optimal performance. This closed-form strategy depends only on the length of the data transmission phase but not on other system parameters such as the relaying energy budget, the number of antennas at the relay, and the distances between the source, relay and destination nodes.This work was supported by the Australian Research Council's Discovery Projects funding scheme (project no. DP0984950, DP110102548) and the Research Council of Norway through the project 197565/V30. The work has been carried out while T. Lamahewa was at the Australian National University

How do hospital doctors manage patients with medically unexplained symptoms: a qualitative study of physicians

OBJECTIVE: Medically unexplained symptoms are a common presentation in medical practice and are associated with significant morbidity and high levels of service use. Most research exploring the attitudes and training of doctors in treating patients with unexplained symptoms has been conducted in primary care. This study aims to explore the ways in which doctors working in secondary care approach and manage patients with medically unexplained symptoms. DESIGN: A qualitative study using in-depth interviews and thematic analysis. SETTING: Three hospitals in the North Thames area. PARTICIPANTS: Twenty consultant and training-grade physicians working in cardiology, gastroenterology, rheumatology and neurology. MAIN OUTCOME MEASURE: Physicians' approach to patients with medically unexplained symptoms and their views on managing these patients. RESULTS: There was considerable variation in how the physicians approached patients who presented with medically unexplained symptoms. Investigations were often ordered without a clear rationale and the explanations given to patients when results of investigations were normal were highly variable, both within and across specialties. The doctor's level of experience appeared to be a more important factor in their investigation and management strategies than their medical specialty. Physicians reported little or no formal training in how to manage such presentations, with no apparent consistency in how they had developed their approach. Doctors described learning from their own experience and from senior role models. Organisational barriers were identified to the effective management of these patients, particularly in terms of continuity of care. CONCLUSIONS: Given the importance of this topic, there is a need for serious consideration as to how the management of patients with medically unexplained symptoms is included in medical training and in the planning and delivery of services

Junior doctors' experiences of managing patients with medically unexplained symptoms: a qualitative study

OBJECTIVES: To explore junior doctors' knowledge about and experiences of managing patients with medically unexplained symptoms (MUS) and to seek their recommendations for improved future training on this important topic about which they currently receive little education. DESIGN: Qualitative study using in-depth interviews analysed using the framework method. SETTING: Participants were recruited from three North Thames London hospitals within the UK. PARTICIPANTS: Twenty-two junior doctors undertaking the UK foundation two-year training programme (FY1/FY2). RESULTS: The junior doctors interviewed identified a significant gap in their training on the topic of MUS, particularly in relation to their awareness of the topic, the appropriate level of investigations, possible psychological comorbidities, the formulation of suitable explanations for patients' symptoms and longer term management strategies. Many junior doctors expressed feelings of anxiety, frustration and a self-perceived lack of competency in this area, and spoke of over-investigating patients or avoiding patient contact altogether due to the challenging nature of MUS and a difficulty in managing the accompanying uncertainty. They also identified the negative attitudes of some senior clinicians and potential role models towards patients with MUS as a factor contributing to their own attitudes and management choices. Most reported a need for more training during the foundation years, and recommended interactive case-based group discussions with a focus on providing meaningful explanations to patients for their symptoms. CONCLUSIONS: There is an urgent need to improve postgraduate training about the topics of MUS and avoiding over-investigation, as current training does not equip junior doctors with the necessary knowledge and skills to effectively and confidently manage patients in these areas. Training needs to focus on practical skill development to increase clinical knowledge in areas such as delivering suitable explanations, and to incorporate individual management strategies to help junior doctors tolerate the uncertainty associated with MUS