A new approach to layered space-time coding and signal processing

The information-theoretic capacity of multiple antenna systems has been shown to be significantly higher than that of single antenna systems in Rayleigh-fading channels. In an attempt to realize this capacity, Foschini (1996) proposed the layered space-time architecture. This scheme was argued to asymptotically achieve a lower bound on the capacity. Another line of work has focused on the design of channel codes that exploit the spatial diversity provided by multiple transmit antennas (Tarokh et al. 1998, Hammons and Gamal 2000). In this paper, we take a fresh look at the problem of designing multiple-input-multiple-output (MIMO) wireless systems. First, we develop a generalized framework for the design of layered space-time systems. Then, we present a novel layered architecture that combines efficient algebraic code design with iterative signal processing techniques. This novel layered system is referred to as the threaded space-time (TST) architecture. The TST architecture provides more flexibility in the tradeoff between power efficiency, bandwidth efficiency, and receiver complexity. It also allows for exploiting the temporal diversity provided by time-varying fading channels. Simulation results are provided for the various techniques that demonstrate the superiority of the proposed TST architecture over both the diagonal layered space-time architecture in Foschini (1996) and the multilayering approach (Tarokh et al. (1999).     

On the theory of space-time codes for PSK modulation

The design of space-time codes to achieve full spatial diversity over fading channels has largely been addressed by handcrafting example codes using computer search methods and only for small numbers of antennas. The lack of more general designs is in part due to the fact that the diversity advantage of a code is the minimum rank among the complex baseband differences between modulated codewords, which is difficult to relate to traditional code designs over finite fields and rings. We present general binary design criteria for PSK-modulated space-time codes. For linear BPSK/QPSK codes, the rank of (binary projections of) the unmodulated codewords, as binary matrices over the binary field, is a sufficient design criterion: full binary rank guarantees full spatial diversity. This criterion accounts for much of what is currently known about PSK-modulated space-time codes. We develop new fundamental code constructions for both quasi-static and time-varying channels. These are perhaps the first general constructions-other than delay diversity schemes-that guarantee full spatial diversity for an arbitrary number of transmit antennas     

Integrating renewable energy sources into European grids

This paper examines the integration of new sources of renewable energy into the power systems in Europe—challenges and possible solutions, application of wind power prediction tools for power system operation, new tasks that create new solutions for communication in distribution systems, wind power in Greece, integration of dispersed generation in Denmark, wind power in The Netherlands, EdF and distributed energy resources in France, and new renewable sources in Italy. The paper also examines the European Commission Technology Platform’s vision paper on Electricity Networks of the Future that was published in January 2006. In this respect, drivers towards Smart Grids, Grids today, and key challenges for Smart Grids of the Future are critically assessed.     

African electricity infrastructure interconnections and electricityexchanges

This paper discusses African electrical energy resources: feasibility studies, interconnection of power systems, the present state of the electric power sector, future expansion of African power systems, and implementation of an African power grid network. It examines involvement of the World Bank with African electricity infrastructure, Grand Inga interconnection projects, the Great Lakes project, and prospects for evaluation of a unified power system in Africa. First, the poorness of African countries and their limited electricity infrastructure is discussed. Objectives for electricity infrastructure development in response to perceived needs is examined and the development challenge for the future, with ways of meeting that challenge defined. Grand Inga interconnections projects are then reviewed. The Democratic Republic of Congo's potential is equivalent to 88400 MW of continuous energy; 42000 MW is located in the Inga region. Development of the Great Lakes interconnected network which links Burundi, Rwanda and Democratic Republic of Congo is also reviewed. The paper discusses existing power systems in Africa and presents the idea of dividing the continent into five main regions of about the same surface area for electricity trade. Feasibility studies of an HVDC line from Democratic Republic of Congo to Egypt and other studies to South Africa and Europe are discussed. A scenario for energy balance for the 2050 is presented, and export of electricity to neighboring continents, mainly Europe, in excess of 200 TWh/year by 2050 is outlined. Implementation of an integrated African grid network is discussed     

Analysis of shaft torsional phenomena in governing large steamturbine generators with non-linear valve stroking

The paper focuses on how shaft torsional problems can be initiated in some situations by the fast action of steam control valves. It demonstrates that shaft flexibility can exert a significant influence on steam turbine-generator response following severe supply system disturbances, particularly for large machines where the turbine has nonlinear valve stroking and fast valving. The effect can be minimised by careful location of the speed sensor on the turbine shaft and by filtering to reduce speed input or acceleration signals due to troublesome low-frequency torsional vibrations to an insignificant level. Digital implementation of steam-turbine control systems is described. Response for a range of control philosophies where digital control procedures are used are given. Two large machines where digital speed sensing and filtering techniques in steam valve control are implemented are examined     

Russia/Germany HVDC Link: Analysis of System Scaling Factors for Evaluating Torsional Stressing of Generators on the Polish Grid System

This paper examines phenomena relating to the effect HVdc link ripple current superimposed on the direct current of an asynchronous link has on torsional stressing of steam turbine-generator-exciter shafts of generators in Poland in close proximity to a proposed 1GW HVdc inverter station. It makes an indepth analysis of dispersion of noncharacteristic harmonic alternating currents, which are injected into the Polish Grid Network by the inverter due to HVdc link ripple current in an asynchronous link. It also investigates the magnitude of ripple current at the inverter due to unbalanced bridge operation (as well as for balanced bridge operation) not analyzed in depth in the literature heretofore.     

Low-threshold proton-implanted 1.3-/spl mu/m vertical-cavity top-surface-emitting lasers with dielectric and wafer-bonded GaAs-AlAs Bragg mirrors

We demonstrate a new structure for long-wavelength (1.3-/spl mu/m) vertical-cavity top-surface-emitting lasers using proton implantation for current confinement. Wafer bonded GaAs-AlAs Bragg mirrors and dielectric mirrors are used for bottom and top mirrors, respectively. The gain medium of the lasers consists of nine strain-compensated AlGaInAs quantum wells. A record low room temperature pulsed threshold current density of 1.13 kA/cm/sup 2/ has been achieved for 15-/spl mu/m diameter devices with a threshold current of 2 mA. The side-mode-suppression-ratio is greater than 35 dB.     

Impact of shaft torsionals in steam turbine control

Shaft torsional vibrations are discussed along with their effect on steam turbine-generator control following severe supply-network disturbances and the destabilization of shaft torsional modes through the action of high-speed electrohydraulic controllers. It is shown that shaft flexibility can exert a significant influence on steam turbine-generator response following a severe supply-system disturbance, particularly when the turbine has nonlinear valve stroking and fast valving. The effect can be minimized by the careful location of a speed sensor along the turbine shaft and by filtering speed or acceleration signals to reduce the speed input of troublesome low-frequency torsional vibrations to an insignificant level. The effect of shaft torsionals on control system response to an islanding situation and on the above mentioned destabilization is evaluated. Digital implementation of steam-turbine control systems in relation to quantization, sampling, response time in protection systems, and software integrity is examined