Opportunistic cooperation in wireless networks

Abstract

Cooperation plays a fundamental role in wireless networks. Many cooperative techniques, such as cooperative diversity, MIMO, and opportunistic routing have been designed and implemented on real networks. However, due to the dynamics of the wireless network, and the lack of information, in many cases, there are only some uncertain opportunities of cooperation. Techniques designed for these cases are known as opportunistic cooperation techniques. Two important questions needed to be answered, about these techniques, are: 1) when to cooperate and 2) whom to cooperate with. Other challenges faced by such techniques are ``on the fly'' decision making, overhead minimization, and etc. In this thesis, these issues are studied in the field of Wireless LANs and Wireless Sensor Networks by applications.

In the area of Wireless LANs, throughput is one of, if not the most, important performance metric. After exploring the opportunity of cooperation in the MAC layer, we propose a new MAC protocol. This is CCMAC, a coordinated cooperative MAC for wireless LANs. It is designed to improve the throughput performance in the region near the AP (a bottleneck area), through cooperative communication. The most unique feature is that, it can coordinate nodes to perform concurrent transmissions, when the opportunities are found. Through analysis and simulation, we show that CCMAC can significantly shorten the transmission time for wireless stations with low data rate link to the AP. It has better throughput performance than other MAC protocols, such as CoopMAC and legacy IEEE 802.11.

In the area of wireless sensor networks (WSN), traditional network routing algorithms can be challenged by nodes' propensities to go to sleep, move around, or even break down. It is costly in terms of communication and energy consumption for routing information to be kept up-to-date. Based on the idea of geographic opportunistic forwarding, we propose a new hybrid opportunistic forwarding protocol: Geographic Multi-hop-Sift (GMS), which combines two opportunistic forwarding techniques: priority list and random access. It is designed to be both energy efficient and robust against channel fluctuation or frequent changes of network topology. In this protocol the next hop relay node is selected by neighboring nodes themselves, using a Sift ``game''. Meanwhile, the sender node can optionally influence the selection process, based on the list of preferred nodes (LPN).

Lastly, a general coordination scheme, based on priority list technique, is proposed. Normally, the overhead caused by coordination is non-negligible for an opportunistic cooperation. The proposed scheme takes both the overhead and the potential benefits into consideration. Based on this scheme, an algorithm with polynomial time complexity is given, to find the best priority list, which can optimize the user-defined metrics.