Vasilis Friderikos

Cross-Layer Optimization to Maximize Fairness Among TCP Flows of Different TCP Flavors

A significant body of recent research has analyzed the problematic behavior of TCP over wireless links, and a plethora of modifications to TCP have been proposed in order to increase its performance in such contexts. Two schools of thought have emerged: the first proposes changes to the end-to- end protocol, while the second explores the potential to enhance lower layers as a means to improve the end-to-end performance of TCP. This paper focuses on the latter, and in contrast to most research in this area, which thus-far has concentrated on a single TCP flavor, examines the case where different TCP flavors are competing over a wireless link. To this end, we present and assess a cross-layer solution that involves the adaptation of lower layer characteristics (i.e., the coding rate) based on the detected TCP flavor, in order to maximize the fairness among TCP flows. Through extensive numerical investigations, we show that the proposed scheme considerably improves the fairness over wireless links among different TCP flavors. Our approach also has a minimal effect on the aggregate throughput of the TCP flows, and in cases where the packet error rate is very low, has a small positive effect on throughput.

Performance analysis of Internet gateway discovery protocols in ad hoc networks

When an ad hoc network is connected to the Internet, it is important for the mobile nodes to detect the available gateways providing access to the Internet. Therefore, a gateway discovery mechanism is required. The two main approaches for discovering Internet gateways are the reactive and the proactive one. This paper compares the performance of these approaches in various scenarios by means of simulation. We show that the proactive approach performs better in the simulated scenario.

Approximate dynamic programming for link scheduling in wireless mesh networks

In this paper a novel interference-based formulation and solution methodology for the problem of link scheduling in wireless mesh networks is proposed. Traditionally, this problem has been formulated as a deterministic integer program, which has been shown to be NP-hard. The proposed formulation is based on dynamic programming and allows greater flexibility since dynamic and stochastic components of the problem can be embedded into the optimization framework. By temporal decomposition we reduce the size of the integer program and using approximate dynamic programming (ADP) methods we tackle the curse of dimensionality. The numerical results reveal that the proposed algorithm outperforms well-known heuristics under different network topologies. Finally, the proposed ADP methodology can be used not only as an upper bound but also as a generic framework where different heuristics can be integrated.

Future Wireless Mobile Networks

In this article, the authors outline some key benefits that stem from the use of mechanical relaying within or across the cells in future wireless mobile networks. The authors have argued that, when information messages that can be stored are elastic enough and subsequently carried by mobile nodes, before relayed to other terminals or the BS at a later time, interference and energy consumption levels can be dramatically decreased during network operation due to the locality of the transmissions. In addition, mechanical relaying allows for innovative resource-management techniques to be deployed in the network, such as providing load balancing or switching-off BSs with low levels of utilization. To fully understand the issues regarding mechanical relaying, it is necessary to examine in a more detailed manner the required architectural changes together with the implementation aspects of integrating the mechanical relaying paradigm with the current and emerging mobile networks such as LTE advanced. This will be part of our future line of enquiry.

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.

Green spectrum management for mobile operators

This paper proposes intra-operator dynamic spectrum access, i.e., dynamic spectrum access by an operators networks of its available spectrum bands, as a means to improve power efficiency. It is based around four areas of interest: (i) dynamically moving users into particularly active bands from other bands to allow radio network equipment in those other bands to be switched off when possible, (ii) the dynamic sharing of spectrum in order to take advantage of better propagation bands and reduce necessary transmission power, (iii) the sharing of spectrum to allow channel bandwidths to be increased thus allowing transmission power to be significantly decreased, and (iv) the better hierarchical management of spectrum in cases where different types of cells coexist. Numerical results show a significant potential for such spectrum management solutions to reduce power consumption for the operator by some 50% or more, with further potential saving if there is a lower correlation in traffic loads among the operators networks/frequencies. It is noted that although the main objective of this paper is power saving, the concepts presented can also be used for other purposes, such as to increase achievable capacity.

Energy-Efficient Relaying via Store-Carry and Forward within the Cell

In this paper, store-carry and forward (SCF) decision policies for relaying within the cell are developed. The key motivation of SCF relaying stems from the fact that energy consumption levels can be dramatically reduced by capitalizing on the inherent mobility of nodes and the elasticity of Internet applications. More specifically, we show how the actual mobility of relay nodes can be incorporated as an additional resource in the system to achieve savings in the required communication energy levels. To this end, we provide a mathematical programming formulation on the aforementioned problem and find optimal routing and scheduling policies to achieve maximum energy savings. By investigating structural properties of the proposed mathematical program we show that optimal solutions can be computed efficiently in time. The tradeoffs between energy and delay in the system are meticulously studied and Pareto efficient curves are derived. Numerical investigations show that the achievable energy gains by judiciously storing and carrying information from mobile relays can grow well above 70 percent for the macrocell scenario when compared to a baseline multihop wireless relaying scheme that uses shortest path routes to the base station.

Robust scheduling in spatial reuse TDMA wireless networks

We propose a framework that produces robust schedules in collision-free medium access schemes. We demonstrate the approach on the STDMA link scheduling problem that seeks to minimize the frame length using the physical interference model and stochastic link gains. By using conservative link gain values as opposed to average values in the SINR-target constraints, we show that the proposed approach produces shorter schedules when timeslots required for retransmission are taken into account. We derive properties on the expected frame length and provide bounds on the probability of SINR constraint violation and on the number of timeslots needed for retransmission.

Cross-Layer Design to Improve Wireless TCP Performance with Link-Layer Adaptation

Transmission control protocol (TCP), the almost universally used reliable transport protocol in the Internet, has been engineered to perform well in wired networks where packet loss is mainly due to congestion. TCP throughput, however, degrades over wireless links, which are characterized by a high and greatly varying bit error rate and by intermittent connectivity. Over such wireless links, the performance achieved by TCP can be improved through the use of cross-layer algorithms at the link-level, which interact with the TCP state machine. In this paper, a TCP-aware dynamic ARQ algorithm is therefore proposed, which utilizes TCP timing information to prioritize ARQ packet retransmissions. Numerical investigation of the proposed algorithm demonstrates the performance improvements that can be attained through this approach, in comparison with TCP-agnostic link-layer approaches.

Switching off low utilization base stations via store carry and forward relaying

Recent research studies on green cellular networking indicate that most of the traffic is serviced by a few cell sites while the rest of the cells remain under-utilized, contributing to significant energy consumption levels. Previous proposals suggest turning off Base Stations (BSs) during low utilization periods to save energy by allowing neighboring cells to increase their coverage to provide the necessary connectivity. In this paper, an alternative (and also complimentary) method is proposed, based on the store-carry and forward (SCF) relaying paradigm that allows BSs with low utilization levels to be switched-off. The proposed SCF relaying scheme exploits the mobility of relay nodes to migrate traffic from BSs of very low utilization to neighbour BSs, allowing in that respect these sites to be switched off. To this end, a joint routing and scheduling problem is formulated to maximize the number of cell sites that can be switched off by also taking into account the communication cost of operating the network. Numerical investigations reveal that significant energy gains can be attained using the proposed SCF relaying scheme for switching off BSs.