Georgios I. Tsiropoulos

Hybrid wireless-broadband over power lines: A promising broadband solution in rural areas

In rural areas, due to the low population density and the poor familiarization with new technologies, new projects related to broadband access are less profitable. In this article a hybrid wireless-broadband over power lines network, suitable for rural and remote areas is presented, offering smart grid applications and broadband access along a 107 km medium voltage power grid in Larissa, a rural area in central Greece. This hybrid solution exploits the ubiquitous presence of the power grid along with the adaptability and accessibility of wireless technology. Through the design and implementation of this hybrid network many aspects of BPL technology were successfully dealt with demonstrating that, in addition to enhancing the power grid performance, W-BPL technology may be considered as an alternative broadband solution in rural and/or sparsely populated areas.

Radio Resource Allocation Techniques for Efficient Spectrum Access in Cognitive Radio Networks

This paper provides an overview of cognitive radio (CR) networks, with focus on the recent advances in resource allocation techniques and the CR networks architectural design. The contribution of this work is threefold. First, a systematic way to study the resource allocation problem is presented; various design approaches are introduced, such as signal-to-interference-and-noise ratio (SINR) or transmission power-based, and centralized or distributed methods. Second, CR optimization methods are presented, accompanied by a comprehensive study of the resource allocation problem formulations. Furthermore, quality of service criteria of the physical or/and the medium access control layers are investigated. Third, challenges in spectrum assignment are discussed, focusing on dynamic spectrum allocation, spectrum aggregation and frequency mobility. Such approaches constitute an emerging trend in efficient spectrum sharing and affect the performance of resource allocation techniques. The open issues for future research in this area are finally discussed, including adaptability-reconfigurability, dual accessibility, and energy efficiency.

Capacity Comparison Between MIMO-NOMA and MIMO-OMA With Multiple Users in a Cluster

In this paper, the performance of multiple-input multiple-output non-orthogonal multiple access (MIMO-NOMA) is investigated, when multiple users are grouped into a cluster. The superiority of MIMO-NOMA over MIMO-OMA in terms of both sum channel capacity and ergodic sum capacity is proved analytically. Furthermore, it is demonstrated that the more users are admitted to a cluster, the lower is the achieved sum rate, which illustrates the tradeoff between the sum rate and maximum number of admitted users. On this basis, a user admission scheme is proposed, which is optimal in terms of both sum rate and the number of admitted users when the signal-to-interference-plus-noise ratio thresholds of the users are equal. When these thresholds are different, the proposed scheme still achieves good performance in balancing both criteria. Moreover, under certain conditions, it maximizes the number of admitted users. In addition, the complexity of the proposed scheme is linear in the number of users per cluster. Simulation results verify the superiority of MIMO-NOMA over MIMO-OMA in terms of both sum rate and user fairness, as well as the effectiveness of the proposed user admission scheme.

Wireless-broadband over power lines networks: A promising broadband solution in rural areas

In rural areas and remote areas the expenditure for new projects related to broadband communications is deterred. In this paper hybrid Wireless-Broadband over Power Lines (W-BPL) technology, suitable for rural and remote areas is presented. This hybrid approach employs BPL technology for the transmission of communication signals via the medium voltage (MV) grid and wireless technology for providing broadband access to end users. The advantages and opportunities of this hybrid solution are presented through the deployment of a trial hybrid W-BPL network in Larissa, a rural area in central Greece. This network offers broadband access and smart grid applications along a 70 km MV power grid. The supported services along with network performance issues are discussed.

Probabilistic call admission control in wireless multiservice networks

A new probabilistic call admission control scheme is proposed for multiservice wireless networks. The new scheme gradually suppresses the admission rate of the new calls and of the calls of each service class (SC) supported considering their priorities independently. The scheme is examined both for a single SC and for multiple SCs under general conditions. The analysis employs Markov chain theory and yields analytical expressions for the call blocking probabilities. The proposed analytical method was validated via simulations employing different distributions for the channel holding time; the simulations demonstrated the accuracy of the proposed framework.

Towards a QoS-Aware IEEE 802.16 downlink sub-frame mapping scheme

IEEE 802.16 (WiMAX) is considered as one of the most promising wireless access technologies supporting high-capacity and long-distance communications as well as user mobility. A problem that should be addressed in the context of multi-access communications is the efficient accommodation of traffic requests to the downlink subframe. The allocation operation in downlink sub-frame is not standardized, while only restrictions on the usage of downlink sub-frames bandwidth are defined. The most noticeable restriction is the rectangular restriction, requiring all downlink allocations to be mapped in a two-dimensions rectangular shaping. This study is a first step towards defining a QoS-aware mapping scheme, prioritizing traffic requests in accordance with the strict delay requirements they pose. The mapping scheme applies horizon scheduling, permitting bursts to be scheduled efficiently and in a simple way, following the horizons as pilots. The QoS-aware mapping scheme is evaluated by means of simulation experiments, which indicate that the proposed scheme operates effectively and efficiently, by reducing the number of unserviced users and traffic requests, and the portion of the dropped real-time traffic.

On the Sum Rate of MIMO-NOMA and MIMO-OMA Systems

In this letter, the performance of non-orthogonal multiple access (NOMA) is compared with conventional orthogonal multiple access (OMA) over multiple-input multiple-output (MIMO) channels. It is proved analytically that for a simple scenario of two users, MIMO-NOMA dominates MIMO-OMA in terms of both sum rate and ergodic sum rate. Furthermore, for a more practical scenario of multiple users, with two users paired into a cluster and sharing a common transmit beamforming vector, the conclusion still holds. Numerical simulations are conducted, which corroborate the analytical findings.

Utility-based probabilistic call admission control for complete fairness in wireless networks

Resource reservation or the other prioritization strategies adopted by Call Admission Control CAC schemes in wireless networks lead to unfair resource allocation to users belonging to different service classes SCs due to high divergence among the respective call blocking probabilities CBPs. In this paper, we propose dynamic optimization of probabilistic CAC P-CAC schemes to assure CAC fairness among users of different SCs in wireless networks. The approach is based on users utility combined with fairness optimization, aiming at dynamically determining the probability value in the P-CAC scheme. This optimal probability is adjusted to network ongoing traffic, CBPs of each SC, prioritization levels characterizing the SCs supported, and the users risk aversion, which reflects their behavior toward the perceived QoS. The existence and uniqueness of the optimal probability that leads to absolute fairness among the users of a wireless network are proven. Copyright

MAC protocols for ultra-wideband ad hoc and sensor networking: A survey

Ultra-wideband (UWB) transmission is an emerging wireless communication technology that is gaining significant interest for future broadband wireless access. This transmission technique based mainly on the Impulse-Radio (IR) paradigm, has the potential to deliver high-speed wireless connectivity at low power and low proximity, characteristics that makes it an ideal choice for wireless sensor networks (WSNs). The distinct physical layer properties call, however, for innovative solutions at the medium access control (MAC) layer. Various MAC protocols with different objectives were proposed for these pulse-based networks. In this article, we first outline the issues related to MAC layer design relying on an IR-UWB physical layer. Then, we describe several MAC protocols emphasizing their strengths and weaknesses. Finally, we identify the cross-layer dependencies between the specified physical and the higher layers of the communication stack and investigate open research issues.

Bandwidth allocation in wireless networks employing social distance aware utility functions

Social networks have been rapidly grown over the last years. The critical socio-technical information they move through may be exploited to improve network performance. This is of utmost importance, especially in Next Generation Networks (NGNs) which provide numerous services with increased bandwidth requirements. In this paper, resource allocation in a bottleneck NGN link is investigated. A utility based technique is proposed where the utility function employed is modified; so as to incorporate the information obtained from an overlaid social network as expressed by an appropriately defined by the users average popularity. Then, the users resource allocation problem is formulated as an optimization problem under inequality constraints. Due to the increased complexity of the problem, the optimal bandwidth allocation is estimated numerically. The relevant results demonstrate the high impact of the social distance parameter on bandwidth allocation.