Athanasios Gkelias

A resource allocation strategy for distributed MIMO multi-hop communication systems

An extended form of multi-hop communication systems is introduced which allows the application of multiple-input-multiple-output (MIMO) capacity enhancement techniques over spatially separated relaying mobile terminals to drastically increase end-to-end capacity. An explicit resource allocation strategy is deduced in terms of fractional bandwidth and power allocations to each relaying hop over ergodic Rayleigh flat fading channels employing orthogonal frequency-division multiple-access (FDMA)-based relaying.

Resource allocation for FDMA-based regenerative multihop links

An approximate but explicit fractional bandwidth and power allocation algorithm for orthogonal regenerative frequency-division multiple-access-based multihop communication systems over ergodic flat-fading channels is presented. The exposed algorithm is assessed for a two-hop scenario, where it is shown to operate within 10% of the optimum resource allocation. The achieved end-to-end capacity is up to 45% higher if compared to a trivial allocation strategy with equal resources.

Utility-proportional fairness in wireless networks

Current communication networks support a variety of applications with different quality of service (QoS) requirements which compete for its resources. This continuously increasing competition highlights the necessity for more efficient and fair resource allocation. Current Network Utility Maximization (NUM) framework fails to achieve this target and alternative approaches cannot operate in networks that consist of wireless links. This paper presents a NUM framework for wireless networks that shares resources according to the utility proportional fairness policy. This policy is shown to prevent rate oscillations in the resource allocation process, allocate resources in a more fair manner among different types of applications and lead to the calculation of closed form solutions for the optimal rate allocation function. Based on this policy, a distributed rate and power allocation algorithm is proposed that gives priority to applications with greater need of resources. Finally, numerical results on the performance of the proposed algorithm are presented and compared against other approaches in the literature.

Capacity of distributed PHY-layer sensor networks

Sensor networks are comprised of nodes with minimal baseband and RF functionalities. In such networks, it is assumed that a source sensor communicates with a target sensor over a number of relaying sensors by utilizing distributed low-complexity space-time encoding techniques, hence the resulting communication scenario is a generalized form of orthogonalized multiple-input multiple-output (MIMO) channels. The contributions of this paper are the derivation of the Shannon capacity in terms of natural units per second per Hertz for such space-time encoded distributed communication scenarios. Closed-form capacity expressions are derived for ergodic flat-fading Rayleigh and Nakagami channels, as well as the communication-rate outage probabilities for aforementioned channels. It is shown that the distributed Alamouti scheme yields the best performance over ergodic channels. In the case of nonergodic channels, the 3/4-rate sporadic space-time block code (STBC) is shown to give optimum performance. Finally, Monte Carlo simulations are used to assess the performance of distributed multistage sensor networks. It is shown that notable power savings can be achieved, compared to the traditional single-link sensor networks.

A new distributed optimization framework for hybrid ad-hoc networks

The continuously increasing demand for resources in modern networks urges for more efficient resource allocation. Such an allocation of resources to network users can be formulated as an optimization problem. However, the existence of wireless links in modern networks and the competition for resources by multimedia applications turn the optimization problem into a non-convex one, which is in general difficult to solve. This paper presents a non-convex optimization formulation to describe the Network Resource Allocation problem in hybrid ad-hoc networks, i.e. networks with both wired and wireless links. To find the optimal solution to this problem, a novel general optimization framework, for non-convex optimization problems, is presented and the necessary and sufficient condition for the convergence of a distributed algorithm to the optimal solution is also proven. Moreover, based on this framework, a distributed joint power and rate adaptation algorithm is proposed to calculate the optimal solution, and finally, the convergence and optimality of the algorithm are verified by simulation.

Average packet delay of CSMA/CA with finite user population

In this paper, a simple closed form solution for the packet delays of the basic carrier sense multiple access with collision avoidance system is derived. Simulation results confirm the applicability and correctness of the derivation.

Impact of Interfering Bluetooth Piconets on a Collocated

In this paper, the effect of cochanneled Bluetooth (BT) piconets on a carrier-sense multiple-access (CSMA)-based wireless local area network (WLAN) is investigated. Specifically, the p-persistent CSMA protocol is considered for WLANs, and the probability of error of a WLAN packet is calculated in the presence of interfering BT packets of different lengths and variable piconet traffic loads and as a function of the BTs frequency-hopping guard time. The probability derivation is then used in conjunction with the p-persistent CSMA throughput and delay formulations to examine its net performance in the presence of BT interference. Simulations have been used to corroborate the analytical results, which indicate that the presence of just one fully loaded interfering BT piconet reduces the peak CSMA throughput by 42%. Furthermore, we show that under fully loaded BT traffic conditions, the effect of more shorter BT packet transmissions on the CSMA delay performance can outweigh the interference impact of a higher number of BT piconets with longer packet transmissions.

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The continuously increasing demand for resources in modern, both wired and wireless, communication networks urges for more efficient resource allocation. Such an allocation of resources to network users can be formulated as an optimization problem. Traditional resource allocation protocols, such as TCP, operate inefficiently in cases that there is competition for resources by multimedia applications and some, or possibly all, links in the network are wireless. In this paper, the performance degradation of TCP in modern networks is quantified to highlight the necessity for a novel optimization-based resource allocation protocol. To this direction, a new optimization framework is presented that can provide the theoretical foundations of such a protocol by proving a sufficient, and in some cases also necessary, condition for distributed solution of non-convex problems. The wide applicability of this general framework is illustrated by considering a resource allocation formulation in TDMA/CDMA ad-hoc networks. The convergence properties to the optimal solution are first identified and a distributed algorithm is proposed. Moreover, a novel heuristic is developed to approximate the optimal solution when the condition does not hold and resolve network oscillations. Finally, the performance of the proposed methodology is evaluated and compared against other approaches in literature by simulation.

-Persistent CSMA-Based WLAN

Wireless mesh networks (WMNs) are expected to support various types of applications with different quality of service (QoS) requirements. Existing works are limited to layered approaches that overlook the interaction between medium access control (MAC) and routing algorithms and often fail to satisfy these requirements in such dynamic wireless environments. The inefficiency of current layered schemes to guarantee these demands has recently triggered the interest for new cross-layered approaches. In this paper, we propose a distributed, multi- constrain, cross-layer QoS routing algorithm for wireless mesh networks that can simultaneous satisfy multiple QoS requirements. Studies with different scheduling algorithms and routing protocols have shown that our algorithm successfully guarantees various QoS requirements and achieves higher network throughput when compared with other standard techniques.

A Non-Convex Distributed Optimization Framework and its Application to Wireless Ad-hoc Networks

Cross-layer routing and scheduling algorithms design for wireless backhaul mesh network has attracted much research interest recently. The network is expected to support various types of applications with different quality of service (QoS) requirements from both routing and scheduling perspectives. Existing works do not efficiently integrate these QoS constraints in route discovery and maintenance phases and overlook the interaction between medium access control (MAC) and routing algorithms. In this work, we propose a novel cross- layer framework of QoS routing and distributed opportunistic scheduling for wireless mesh network, which provides resource reservation for QoS flows. Studies with different scheduling algorithms and routing protocols have shown that our algorithm successfully guarantees various QoS requirements and achieves higher network throughput when compared with other standard techniques.