Performance of UWB Receivers with Partial CSI Using a Simple Body Area Network Channel Model

Ultra wideband (UWB) communication is a very promising candidate for the use in wireless body area networks (BAN). The high UWB peak data rate allows for medium average data rates in combination with a very low duty cycle, which is the key for a very low power consumption. Devices in a wireless BAN require low complexity. Hence, mainly non-coherent receivers such as energy detector and transmitted-reference receiver are suited. In this paper, the symbol-wise maximum-likelihood (ML) detectors for pulse position modulation (PPM) and transmitted reference pulse amplitude modulation (TR PAM) are derived assuming partial channel state information (CSI) at the receiver. Additionally, also the ML detectors for a combination of PPM and TR PAM are presented. The performance of the derived receiver structures is evaluated using a novel BAN channel model not distinguishing line-of-sight and non line-of-sight situations. This simple channel model is based on 1100 channel measurements in the frequency range between 2 and 8 GHz, which were measured in an anechoic chamber. Using the BAN channel model, performance of the derived receiver structures is evaluated showing that the knowledge of the average power delay profile (APDP) at the receiver improves performance substantially. Requiring only slightly more complexity such receivers are a well suited alternative to non-coherent receivers for the use in a BAN.     

Recent advances in amplify-and-forward two-hop relaying [Accepted from Open Call]

In this article we review an important class of wireless cooperation protocols known as amplify-and-forward relaying. One or more low-complexity relay nodes assist the communication between sources and destinations without having to decode the signal. This makes AF relaying transparent to modulation and coding of the source/destination communication protocol. It is therefore a highly flexible technology that also qualifies for application in heterogeneous networks comprising many nodes of different complexity or even standards. Recently, two-way relaying was introduced, which is readily combined with AF relaying. It is a spectrally efficient protocol that allows for bidirectional communication between sources and destinations. In order to investigate the potential of wireless AF relaying, we introduce three distributed network scenarios that differ in the amount of cooperation between nodes. New challenges arise in those networks, and we discuss approaches to overcome them. For the most general case of a completely distributed system, we present coherent relaying solutions that offer a distributed spatial multiplexing gain even for single-antenna nodes. Based on real-world experiments, we validate the feasibility of all schemes in our laboratory.     

Exxon is right: Let us re-examine our choice for a cap-and-trade system over a carbon tax

This commentary examines the impact of the recently launched European Union Emission Trading System (EU ETS) in terms of emission reductions and cost to the public. The study points out that a cap-and-trade system may not be the most cost-efficient mechanism to reduce greenhouse gas emissions. It also lists seven main differences between such a system and a carbon tax along the following issues: amount of emissions reduced; flow of revenue to the public purse; cost of the system to the public; marginal cost of carbon emission reductions to the firm; generating excess rent; price setting mechanism and stability of system; as well as duration and commitment. When looking at emission reductions along these dimensions, it becomes clear that an internationally coordinated carbon tax may be a quicker and cheaper way to reduce greenhouse gas emissions worldwide.     

The Cellular Relay Carpet: Distributed Cooperation with Ubiquitous Relaying

We consider the up- as well as downlink of a cellular network in which base stations (BSs) are supported by a large amount of relays spread over the entire area like a carpet. The BSs only see the static relays as the nodes they communicate with, which enables large antenna arrays at the BSs with sophisticated multi-user MIMO transmission. Together with a simple form of BS cooperation, the communication via the small relay cells allows to improve the data rates by distributed interference management and to reduce the complexity at the terminals. We investigate different types of relays as well as different relaying strategies for this relay carpet and compare them with respect to complexity, required channel state information (CSI), and performance in the interference-limited environment of dense cellular networks. The robustness of the different schemes with respect to channel estimation errors is studied and we conclude that especially relays of very low complexity are not sensitive to CSI imperfections. Relays can thus be applied in large numbers and enable massive MIMO at the BSs. The relay carpet proves thereby to be an efficient approach to enhance future generations of cellular networks significantly.     

Power Allocation Schemes for Amplify-and-Forward MIMO-OFDM Relay Links

We consider a two-hop MIMO-OFDM communication scheme with a source, an amplify-and-forward relay, and a destination. We examine the possibilities of power allocation (PA) over the subchannels in frequency and space domains to maximize the instantaneous rate of this link if channel state information at the transmitter (CSIT) is available. We consider two approaches: (i) separate optimization of the source or the relay PA with individual per node transmit power constraints and (ii) joint optimization of the source and the relay PA with joint transmit power constraint. We provide the optimal PA at the source (or the relay) with a node transmit power constraint that maximizes the instantaneous rate for a given relay (or source) PA. Furthermore, we show that repeating this separate optimization of the source and the relay PA alternately converges and improves the achievable rate of the considered link. Since the joint optimization of the source and the relay PA is analytically not tractable we use a high SNR approximation of the SNR at the destination. This approximation leads to rates which are quite tight to the optimum.     

An efficient baseband modem technique for data transmission overanalog FM links

In mobile radio where data are transmitted over existing analog FM systems, the receive bandpass bandwidth, which is adapted to the analog speech transmission, is larger than would be required by data transmission. This results in poor error performance. A novel baseband modem technique which drastically improves the error performance is proposed and analyzed. A smearing filter is used to convert the compound baseband noise at the limiter discriminator output to approximately Gaussian noise. This optimizes the performance at large carrier-to-noise ratios (CNRs). A baseband click detection and elimination scheme, which improves the performance at small CNR is proposed. Simulation results show that this system requires 3.8 dB less CNR than the conventional digital FM system to achieve a bit error probability of 10^-4. It is concluded that the technique is attractive for data transmission over analog FM links     

Spectral efficient protocols for half-duplex fading relay channels

We study two-hop communication protocols where one or several relay terminals assist in the communication between two or more terminals. All terminals operate in half-duplex mode, hence the transmission of one information symbol from the source terminal to the destination terminal occupies two channel uses. This leads to a loss in spectral efficiency due to the pre-log factor one-half in corresponding capacity expressions. We propose two new half-duplex relaying protocols that avoid the pre-log factor one-half. Firstly, we consider a relaying protocol where a bidirectional connection between two terminals is established via one amplify-and-forward (AF) or decode-and-forward (DF) relay (two-way relaying). We also extend this protocol to a multi-user scenario, where multiple terminals communicate with multiple partner terminals via several orthogonalize-and-forward (OF) relay terminals, i.e., the relays orthogonalize the different two-way transmissions by a distributed zero-forcing algorithm. Secondly, we propose a relaying protocol where two relays, either AF or DF, alternately forward messages from a source terminal to a destination terminal (two-path relaying). It is shown that both protocols recover a significant portion of the half-duplex loss     

Working set measurements based on sampled reference string information

The development of new memory technologies combined with the pressures for ever larger, database software systems will continue to make a multi-level memory an attractive and cost-effective system organization. The performance of a process executing in a system with such a memory hierarchy depends on both the memory referencing behavior of the process and the policy used to control the contents of each memory level. The direct observation of this referencing behavior by capturing every memory reference involves a substantial overhead and can not be applied to ‘time-critical’ systems because the measurement radically perturbs the normal operation of the system. This paper reports on the development and testing of a technique which determines the structural characteristics of a process's memory referencing behavior based only on a sampling of the complete reference string. This method controls the cost of the measurement by adjusting the sampling rate while simultaneously attempting to accurately determine the referencing behavior as reflected in its working set measurements. The results presented in this paper are based on the use of a set of successively more realistic synthetic behavior models as well as on actual reference strings.     

Power line enhanced cooperative wireless communications

In this paper, we investigate the use of power line communication (PLC) to assist cooperative wireless relaying. We consider a communication scheme that uses the power line to initialize and synchronize wireless amplify-and-forward relays and to broadcast information between the relays. Starting from an analysis of transfer functions and noise measurements of PLC channels in office and residential environments, we propose a power line transmission scheme for the inter-relay-communication and assess the influence of this scheme on wireless relaying. This scheme is based on linear precoded orthogonal frequency-division multiplexing; it is designed to optimally exploit the frequency diversity available on PLC channels. The use of PLC leads to a very flexible way of enhancing wireless communications by plugging in additional relays where they are needed-without additional wiring.     

Lining design for the district heating tunnel in Copenhagen with steel fibre reinforced concrete segments

The 3.9 km long district heating tunnel carries heating pipelines from a power plant on the island Amager into the centre of Copenhagen. A shaft on Amager and two shafts in the city centre provide access to the tunnel, which is located within the Copenhagen limestone at a depth between 25 and 38 m below the ground surface. The tunnel is excavated with an earth pressure balance (EPB) shield machine and is lined with steel fibre reinforced concrete (SFRC) segments without any conventional steel bar reinforcement. This represents an advantageous and innovative technical as well as cost-efficient solution. This paper presents the methods applied in the structural and durability design of the SFRC segmental lining for a specified service life of 100 years. Special attention is required due to the temperature increase in the tunnel to approximately 50 °C during operation of the pipelines. The experiences with the SFRC segments during construction of the tunnel are discussed.