Improving Throughput and Minimizing Age of Information in dense WLANs, Using Cooperative Techniques

Abstract: Mobile and wireless data are in increasing demand worldwide. New trends such as the Internet of Things paradigm and the Smart City paradigm describe scenarios comprising thousands of devices all exchanging information amongst themselves wirelessly --- or through the WAN to another device, possibly connected to another WLAN. Operators and radio engineers are faced with the problem of designing efficient ways to share the electromagnetic spectrum --- a scarce and expensive resource --- between thousands of devices.In this context, operators look at the unlicensed spectrum as a possible solution to complement the existing infrastructure. Unfortunately, the IEEE 802.11 MAC family, the most widespread MAC protocol in the unlicensed portion of the spectrum, still suffers when managing a large number of interconnected devices. In this thesis we are both addressing the open problems in the IEEE 802.11 MAC scheme and our contributions on their solution.Specifically, in the first part of the thesis we will present the IEEE 802.11 MAC scheme and the challenges it faces, along with solutions already present in literature. We will also show a new metric recently defined in the literature called the Age of Information (AoI). This new metric is a measure of how fresh the piece of information stored in a remote receiver is. Age of Information attracted interest in the literature, but little is known about how it behaves in a IEEE 802.11 WLAN.In the second part of the thesis we present two papers and an appendix that address the problem of designing new protocols that let the devices cooperate in order to achieve a common goal. Specifically, these papers focus on two metrics. The first paper addresses collision reduction and throughput via a new MAC scheme that uses RSSI to identify other devices in a WLAN, and uses a priority based access system in order to act cooperatively. We show, through simulation, that this scheme outperforms the classical IEEE 802.11 DCF mode of operation, especially in WLANs subject to high loads.The second paper addresses the AoI both in terms of average and variance, for sensor nodes embedded in a dense WLAN that send pieces of information to a remote server via a WAN connection. We study both those metrics for a link with high variance and low variance delay. We construct and test, via means of simulations, an AoI-aware MAC, called LUPMAC --- Latest Update Medium Access Scheme, aimed at reducing both the average AoI and the AoI variance at the remote server side, and is also resilient to variations on the wired remote connection.In the appendix we present an analytical continuation of the second paper; we calculate the analytical probability of removal due to staleness of the packet in a new cooperative MAC scheme for Wireless Sensor Networks (WSNs) called COOPLUP --- COOperative LUPMAC. This protocol is aimed at decreasing the number of transmissions in a WSN with sensors broadcasting updates about a measured phenomenon, while minimizing the average AoI at the receiver.In these two papers and appendix we present three schemes suitable for the unlicensed spectrum environment, addressing both scheduling and queuing policies. These schemes are only slight modifications to the already widely deployed IEEE 802.11 MAC, but they significantly improve the metrics they focus on. They rely only lightly on a centralized unit, as most random access schemes do, but instead let the devices cooperate to a certain extent in order not to pollute the channel with undesired retransmissions.