Flow‐level performance analysis of a multi‐rate system supporting stream and elastic services

We consider a multi‐rate system where stream and elastic flows receive service. Both the stream and the elastic classes are associated with peak rate limitation. In contrast to the constant bit rate stream flows, the elastic flows tolerate bandwidth compression while in service. Because of the occasional bandwidth compression, the holding time of elastic flows depends on their perceived throughput. Although this model is Markovian under quite non‐restrictive assumptions, the model's state space grows exponentially with the number of traffic classes. The model is not quasi‐reversible, and therefore, it cannot be evaluated by efficient recursive formulae. We propose a method whereby the original state space is mapped to a two‐dimensional one, independently of the number of the stream and the elastic traffic classes. The special structure of the two‐dimensional model allows us to develop an efficient method that approximates the average throughputs of elastic flows. The state space reduction together with the proposed approximation provides a powerful tool for the performance analysis of this model as it allows the approximation of the average throughputs of elastic flows reasonably accurately in large models as well. Copyright © 2012 John Wiley & Sons, Ltd.     

Performance analysis of cellular CDMA high‐speed data services

This paper investigates the forward‐link peak and average data rates, throughput, and coverage of a cellular CDMA system for delivering high‐speed wireless data services. The analysis takes into account major aspects commonly found in the forward data channel and applies the generalized Shannon capacity formula for multi‐element antenna (MEA) systems. The study focuses on the physical layer and is flexible for various propagation environments, antenna configurations, multicode allocations, user distributions, and cell site configurations. Numerical results for various multicode allocations are presented for a system model with two‐tier interfering cells operating under a frequency selective slow fading channel with propagation environments specified in the Recommendation ITU‐R M.1225. Copyright © 2006 John Wiley & Sons, Ltd.     

Energy payback time of the high‐concentration PV system FLATCON®

The ecological benefit and sustainability of a new energy technology and its potential to reduce CO₂ emissions depend strongly on the amount of energy embodied in the materials and production processes. The energy payback time is a measure for the amount of time that a renewable energy system has to operate until the energy involved in its complete life‐cycle is regenerated. In this paper, the energy payback time of the high‐concentration photovoltaic system FLATCON® using III–V semiconductor multi‐junction solar cells has been evaluated. Considering the energy demand for the system manufacturing, including transportation, balance of system and system losses, the energy payback time turns out to be as low as 8–10 months for a FLATCON® concentrator built in Germany and operated in Spain. The energy payback time rises slightly to 12 to 16 months for a system installed in Germany. The main energy demand in the production of such a high‐concentration photovoltaic system was found to be the zinced steel for the tracking unit. Copyright © 2005 John Wiley & Sons, Ltd.