Open Topics

Vol. 68, n° 5-6, May-June 2013
Content available on Springerlink

Simple precoding algorithms using Gram–Schmidt orthonormalization process for multiuser relay communications with optimized power allocation

Hamid Meghdadi, Jean-Pierre Cances and Vahid Meghdadi 
University of Limoges, France

Abstract This paper proposes a new simple precoding solution based on the Gram–Schmidt orthonormalization to be used at the relay station of a multirelay wireless networks where the different mobile stations belong to the same network, in order to mitigate the multiuser interference at each mobile station. The strength of this method is that it only requires the knowledge of all channel impulse responses from a given relay to all the mobile stations. In other words, to compute its precoding vectors, each relay does not need to know the channel impulse responses of the channels of other relays. Unlike the centralized reference method where each mobile station benefits from the same diversity gain, using this algorithm, some mobile stations will improve their diversity gain at the cost of a loss in the diversity gain of other users. This constitutes a simple solution to supply different qualities of service in the case of a multiservices network. Furthermore, this work proposes an optimized power allocation between the relays. Analytical and accurate performance analyses for the different studied contexts are provided.

Keywords Cooperative diversity gain – Gram–Schmidt algorithm – Multiuser interference mitigation – Expectation maximization – Power allocation optimization.

Adapting contention-based forwarding to urban vehicular topologies for active safety applications

Fatma Hrizi1, Christian Bonnet1, Jérôme Härri1, and Fethi Filali2
1 Eurecom, Sophia-Antipolis, France
2 QUWIC, Doha, Qatar

Abstract Contention-based forwarding (CBF) is a broadcasting technique used to disseminate emergency messages for traffic safety applications in intelligent transportation systems. Its design hypotheses have however been based on three major assumptions: uniform vehicular topology, nonfading channels and homogeneous communication capabilities. Realistic vehicular urban topologies do not comply with any of them, making CBF select relays, which may not exist, may not be reached or may not be optimal due to heterogeneous transmit capabilities. In this paper, we propose to adapt CBF to such challenging environment by first employing two different mechanisms as a function of the topology and second by considering the dissemination capabilities of the relays, allowing for example road-side units or tall vehicles to preferably act as relays when necessary. Our protocol, called Bi-Zone Broadcast, is evaluated in a realistic urban environment and showed to provide around 46 % improvement in dissemination delay and 40 % reduction in overhead compared to plain CBF or flooding. We finally shed light to other aspects of CBF that remain unsolved and should be addressed in future work to further improve the reliability of dissemination protocols for traffic safety protocols.

Keywords Dissemination protocol – Broadcast Contention-based forwarding – Vehicular urban topology – Performance evaluation – Traffic safety applications – Intelligent transportation systems

Cross-layer multiple-access optimization scheme for linear precoded OFDM UWB systems

Ayman Khalil1,2, Matthieu Crussière1,2 and Jean-François Hélard1,2
European University of Brittany (UEB), INSA, Rennes, France
Institute of Electronics and Telecommunications of Rennes (IETR), France

Abstract With the growing demand for new wireless applications accompanied with high expectations for better QoS fulfillment especially for multimedia and real-time applications, the performance of the radio resource management in a multiuser context is ensured by the ability to provide an efficient and optimized spectrum sharing scheme that should respect the wireless channel conditions and satisfy the different users’ demands. From the physical layer perspective, metrics such as spectrum efficiency and minimum BER are the most important criteria to be considered. On the other hand, from a user perspective, QoS as well as fairness among the competing users are the main metrics because they determine how much end-users are satisfied and how efficient the available resources are shared among the existing users. Based on the use of the linear precoded orthogonal frequency division multiplexing (LP-OFDM) solution proposed as an evolution of the well-known multiband OFDM (MB-OFDM) solution supported by the WiMedia Alliance for future high-rate ultra-wideband (UWB) systems, the objective of this paper is twofold. First, we study a multiuser optimization spectrum sharing scheme for LP-OFDM systems. Second, based on the optimization study, we define a novel multiple-access solution which jointly considers the frequency resource allocation and the time scheduling for the high-rate LP-OFDM UWB systems. Simulation results demonstrate the efficiency of the use of the LP-OFDM transmission technique in the multiuser spectrum sharing scheme. Besides, the novel multiuser time–frequency sharing scheme shows its capacity to provide a high performance level for high-priority users.

Keywords Multiple-access – Quality of Service (QoS) – Time frequency LP-OFDM

Minimization in variations of different parameters in different φ planes of a small-size concentric ring array antenna using firefly algorithm

A. Chatterjee and G. K. Mahanti
National Institute of Technology, Durgapur, India

Abstract The radiation pattern of a small-size concentric ring array antenna does not remain φ-symmetric, if the array is made with an appreciably lesser number of antenna elements. The sidelobe level, first null beamwidth (FNBW), half-power beamwidth (HPBW), and the null depths of the radiation pattern change significantly in different φ planes. An efficient pattern synthesis method has been developed in this work to reduce the variation of sidelobe level, FNBW, and HPBW of a small-size concentric ring array of isotropic antennas, for different φ cuts. It is done by finding out an optimum set of amplitude distribution of the array elements using firefly algorithm. The sidelobe level and the first null depth of the array are kept below a desired value for all the defined φ cuts. The directivity of the optimized array and the uniform array has been computed. The variation of all these parameters for different φ cuts has been compared to the same array with uniform excitation among the elements.

Keywords Circular ring array – Directivity Firefly algorithm (FA) – First null beamwidth (FNBW) – Half-power beamwidth (HPBW) – Sidelobe level

Modeling the user acceptance of long-term evolution (LTE) services

Eunil Park1 and Angel P. del Pobil1,2
1 Sungkyunkwan University, Seoul, South Korea
2 University Jaume-I, Castellon, Spain

Abstract With an integrated framework, this paper aims to analyze user perception and acceptance toward long-term evolution (LTE) services, focusing on factors that may influence the intention to use. We conducted a web-based survey of 1,192 users to test our research model. We employed structural equation modeling (SEM) as the analysis method. The results of the integrated model analysis indicate that system satisfaction is a core determinant of intention to use LTE services. The model also found that other factors, including perceived usefulness and system and service quality, significantly affect intention to use these services. In addition, both perceived adaptivity and processing speed significantly influence perceived usefulness and service quality, respectively. These factors also play key roles in determining users’ attitudes. This paper is of value to researchers and engineers designing and improving LTE services for use via mobile phones.

Keywords Long-term evolution (LTE) – Technology acceptance – Service quality – System satisfaction

Mitigation of channel estimation error in TR–UWB system based on a novel MMSE equalizer

H. Khaleghi Bizaki and S. Alizadeh
Electrical and Electronic Engineering University Complex (EEEUC), Tehran, Iran

Abstract The time reversal (TR) technique combined with the ultra-wideband (UWB) system offers a new potential for decreasing the cost and complexity of the UWB receivers. In spite of TR–UWB’s good performance in perfect channel state information (CSI), it is very sensitive to the channel estimation error. The effect of channel imperfection on the TR–UWB system is considered in this paper. At first, based on a minimum mean square error (MMSE) equalizer receiver, a prefilter is calculated in closed form to improve the performance of the TR–UWB system in an imperfect CSI scenario. Furthermore, for comparison purposes, a similar calculation for prefilter is carried out based on a simple matched filter (MF) receiver. Then, in order to improve the MF receiver performance, a two-stage iteration-based algorithm is developed. The initial value for this iteration-based improved algorithm is considered to be a prefilter which is calculated in the TR–UWB system with MMSE equalizer. This optimized algorithm causes the channel estimation error in the TR–UWB system to become zero in some steps. Finally, exhaustive simulations are done to demonstrate the performance advantage attained by the improved algorithm.

Keywords Temporal focusing – Time reversal (TR) technique – Ultra-wideband (UWB) system -Channel estimation error – Matched filter – Prefilter – MMSE equalizer

Congestion probabilities in a batched Poisson multirate loss model supporting elastic and adaptive traffic

Ioannis Moscholios1, John Vardakas2, Michael Logothetis2 and Anthony Boucouvalas1
1 University of Pelopponese, Tripolis, Greece
2 University of Patras, Greece

Abstract The ever increasing demand of elastic and adaptive services, where in-service calls can tolerate bandwidth compression/expansion, together with the bursty nature of traffic, necessitates a proper teletraffic loss model which can contribute to the call-level performance evaluation of modern communication networks. In this paper, we propose a multirate loss model that supports elastic and adaptive traffic, under the assumption that calls arrive in a single link according to a batched Poisson process (a more “bursty” process than the Poisson process, where calls arrive in batches). We assume a general batch size distribution and the partial batch blocking discipline, whereby one or more calls of a new batch are blocked and lost, depending on the available bandwidth of the link. The proposed model does not have a product form solution, and therefore we propose approximate but recursive formulas for the efficient calculation of time and call congestion probabilities, link utilization, average number of calls in the system, and average bandwidth allocated to calls. The consistency and the accuracy of the model are verified through simulation and found to be quite satisfactory.

Keywords Batched Poisson process – Elastic–adaptive traffic – Recursive formula – Time–call congestion – Markov chain

Multicellular Alamouti scheme performance in Rayleigh and shadow fading

Dorra Ben Cheikh1,2, Jean-Marc Kelif1, Marceau Coupechoux2 and Philippe Godlewski2
1 Orange Labs, Issy-les-Moulineaux, France
2 Télécom ParisTech, CNRS, Institut Mines-Téélcom, France

Abstract In this paper, we study the performance of two downlink multicellular systems: a multiple inputs single output (MISO) system using the Alamouti code and a multiple inputs multiple outputs (MIMO) system using the Alamouti code at the transmitter side and a maximum ratio combining (MRC) as a receiver, in terms of outage probability. The channel model includes path-loss, shadowing, and fast fading, and the system is considered interference-limited. Two cases are distinguished: constant shadowing and log-normally distributed shadowing. In the first case, closed form expressions of the outage probability are proposed. For a log-normally distributed shadowing, we derive easily computable expressions of the outage probability. The proposed expressions allow for fast and simple performance evaluation for the two multicellular wireless systems: MISO Alamouti and MIMO Alamouti with MRC receiver. We use a fluid model approach to provide simpler outage probability expressions depending only on the distance between the considered user and its serving base station.

Keywords Alamouti scheme – MISO – MIMO – MRC – Multicellular – Outage probability – Shadowing – Rayleigh fading – Fluid model