Beatrice Paillassa

Improving satellite services with cooperative communications

SUMMARY This paper proposes new transmission schemes for the delivery of satellite services. In the proposed scenarios, mobile terminals are allowed to forward the signal received from the satellite. This scheme provides spatial diversity just like MIMO transmission schemes. Moreover, the coverage area is extended because masked terminals have an additional opportunity to get the service from neighboring terminals. We use the paradigm of cooperative communications to compare the advantages and limitations of several scenarios in hybrid terrestrial/satellite systems. In particular, we study the following basic transmission scheme: in a first time slot, the satellite sends its signal and, in a second time slot, mobile terrestrial terminals are relaying the satellite signal. An analysis framework is proposed and applied to this cooperation scenario at the destination terminal. The framework is modeling the cooperation process and clearly separates the control part from the data user part. The paper outlines the importance of the control part by evaluating the relay selection policy on a basic hybrid satellite/ad hoc system. Copyright

Mobility Metrics Evaluation for Self-Adaptive Protocols

Cross-layer mechanism, for which a protocol locating at a given layer uses information issued from other layers, may enhance the mobile networks performance. Some of those mechanisms are based on mobility metrics. For example, the establishment of a route by choosing less mobile nodes could improve the routing protocol. In this paper, we study the ability of mobility metrics to reflect the mobility influence over the protocol performances. The proposed approach evaluates the ability of a metric from its capacity to indicate or predict the routing protocol performance. Three routing protocols are considered: AODV, DSR and OLSR. The studied mobility metrics are Frequency of Link State Changes (LC), Link Connectivity Duration (LD) and Link Stability Metric (LS). The metrics are evaluated by simulation, firstly in a general case then in a scenario case.

Towards generic satellite payloads : software radio

Satellite payloads are becoming much more complex with the evolution towards multimedia applications. Moreover satellite lifetime increases while standard and services evolve faster, necessitating a hardware platform that can evolve for not developing new systems on each change. The same problem occurs in terrestrial systems like mobile networks and a foreseen solution is the software defined radio technology. In this paper we describe a way of introducing this concept at satellite level to offer to operators the required flexibility of the system. The digital functions enabling this technology, the hardware components implementing the functions and the reconfiguration processes are detailed. We show that elements of the software radio for satellites exist and that this concept is feasible.

Network awareness and dynamic routing: The ad hoc network case

For extremely dynamic networks, such as ad-hoc, the topology change awareness has a crucial impact on the routing performance and consequently many routing protocols adapt their processes to the state of the network, from some network awareness. While several works have already been done on routing adaptations, this paper is more focused on the network awareness topic and the choice of the best metrics for a given adaptation. More precisely, the paper considers the way to represent by means of metrics the node mobility, the link degradation or the graph topology. The notion of metrics is illustrated through two adaptations of two well known ad hoc routing protocols (DSR,OLSR). We evaluate the effect of different metric choices by considering several adaptation strategies to the topology change which are based on the awareness of both the node movement and the number of nodes. We analyze the adaptation strategies and evaluate the performance of the adaptation depending on the chosen metrics. It is shown that the performance of adaptation is strongly correlated to the metrics that are themselves correlated to the network size. A metric combination based on link duration and number of nodes is found to be a good way to represent the topology change.

A framework for cooperative communications at the system level

In order to improve the communication efficiency without using multi-antennae terminals, cooperative communications implement one or several single antenna relays to assist the transmission between a source and a destination. The design of such a cooperative network, involves several layers of the OSI model. Transmission and multiplexing techniques are addressed at the physical layer whereas the management of the cooperative network (activation of a cooperative mode, selection of relays) is done at the Medium Access Control (MAC) layer and the layers above. After an overview of protocols that have been designed in this domain, we propose an original framework of a cooperative network, at the system level. Inspired from models of the ITU normative organization, our cooperative network model is based on two planes: a data plane and a control plane. A validation of the framework is given by modeling an existing cooperative MAC protocol.

Impact of SDN on Mobility Management

The software integration with new network architectures via SDN (Software Defined Network) axis appears to be a major evolution of networks. While this paradigm was primarily developed for easy network setup, its ability to integrate services has also to be considered. Thus, the mobility service for which solutions have been proposed in conventional architectures by defining standardized protocols should be rethought in terms of SDN service. Mobile devices might use or move in SDN network. In this paper, we have shown that SDN can be implemented without IP mobility protocol for providing mobility like as Proxy Mobile IPv6 (PMIPv6) that is the solution adopted by 3GPP, with some performance gain.

On Metrics for Mobility Oriented Self Adaptive Protocols

Mobile networks performance may be enhanced by cross-layer mechanisms. Some of those mechanisms are based on mobility metrics. For example, the establishment of a route by choosing less mobile node could improve the routing protocol. In this paper, we study the ability of mobility metrics to reflect the mobility. The proposed approach evaluates the ability of a metric from its influence over routing protocols. Three routing protocols are considered: AODV, DSR and OLSR. The studied mobility metrics are frequency of link state changes (LC), link connectivity duration (LD) and link stability metric (LS). The metrics are evaluated by simulation, firstly in a general case then in a scenario case.

Experimental Study on TCP Enhancement Interest in Ad Hoc Networks

As TCP performance over ad hoc networks is degraded, many enhancements have been proposed in the literature. We classify them as: intra layer, under layer and cross layer enhancements. We examine by simulation the interest to optimize the flow control mechanism and analyze that, compare to the routing protocol impact, the flow control one over the performance is marginal. Simulation results of a cross layer method, which is representative of enhancements recently proposed, are then examined. The cross layer mechanism is based on a freezing process triggered by a route failure event. With this type of optimization, the performance gain can be important but it is strongly dependent on the network mobility and in some cases it can work worst than TCP. From the TCP optimization analysis it appears that a mobile oriented self adaptive mechanism would be profitable.

On routing for extending satellite service life in LEO satellite networks

We address the problem of routing for extending the service life of satellites in Iridium-like LEO constellations. Satellites in LEO constellations can spend over 30% of their time under the earths umbra, time during which they are powered by batteries. While the batteries are recharged by solar energy, the depth of discharge they reach during eclipse significantly affects their lifetime - and by extension, the service life of the satellites themselves. For batteries of the type that power Iridium satellites, a 15% increase to the depth of discharge can practically cut their service lives in half. We present two new routing metrics - LASER and SLIM - that try to strike a balance between performance and battery depth of discharge in LEO satellite constellations. Our basic approach is to leverage the deterministic movement of satellites for favoring routing traffic over satellites exposed to the sun as opposed to the eclipsed satellites, thereby decreasing the average battery depth of discharge - all without adversely affecting network performance. Simulations show that LASER and SLIM can reduce the depth of discharge by about 11% and 16%, respectively, which can lead to as much as 100% increase in the satellite batteries lifetime. This is accomplished by trading off very little in terms of end-to-end delay.

Mobility Versus Density Metric for OLSR Enhancement

In order to improve network performance, adaptive protocol would adapt to different aspects of the network dynamic exhibited by the wireless systems and more particularly by the ad hoc networks. In this paper we consider the adaptation to the ad hoc network dynamic through two parameters: mobility and density. We study the impact part of the density metric and of the mobility metric. Considering the Optimized Link State Routing protocol (OLSR), our work focus on the Multipoint Relays (MPR) selection. A new approach to select a MPR by using a simple modification and no additional packet header is proposed. It introduces the idea of Link Duration criterion as mobility metric for MPR selection. From simulation results it appears that the protocol performance can be enhanced by mobility adaptation after the density one. The proposed scheme outperforms the standard protocol for large number of nodes.