Cross-layer Based Cooperative Communication And Multi-channel MAC Protocols For Wireless Ad Hoc Networks

Wireless ad hoc network requires no infrastructure and is characterized of agility,quick organization, and easy extention. Therefore, wireless ad hoc network has gainedwidespread attention and study by industry. However, with the popularity of wirelessbroadband services, traditional wireless ad hoc network has difficulty to meet thegrowing needs of network users. In order to alleviate this contradiction, newtechnologies are needed to promote the development and application of wireless ad hocnetworks. Recently proposed cooperative communication technology and multi-channeltechnology provide new potentials to enhance the performance of wireless ad hocnetworks. From this point of view, this dissertation focuses on introducing both thecooperative communication technology and the multi-channel technology into wirelessad hoc network to improve the system performance.This dissertation first analyzes the key aspects of cooperative MAC design, namely,when to cooperate, whom to cooperate with and how to cooperate. In order to reap morebenefit of cooperation, a cross-layer adaptive cooperative MAC (CAC-MAC) protocolfor IEEE802.11DCF based wireless ad hoc networks is proposed, which involvesinteraction between MAC layer and physical layer. The key features of our proposal areas follows. First, each node adjusts its transmission rate according to the instantaneouswireless channel conditions. Second, only when a data frame at the MAC layer is longerthan a specified length, CAC-MAC initiates a RTS/CTS handshake, which brings downthe overhead of network. And for long data frames, RTS/CTS direct transmission orproperly cooperative transmission will be selected according to the wireless channelconditions. Moreover, the cooperative transmission is divided into either“source-relay-destination” transmission scheme or receiver maximal ratio combiningscheme according to the channel conditions among source, relay and destination. Third,the best relay node for a given source-destination pair is selected based on instantaneouswireless channel measurements instead of a relay table that used in traditional ways,which can cut the cost to create and maintain the raly table. Analysis and simulationresults show that the CAC-MAC protocol can significantly improve network throughputand reduce packet delay compared with legacy IEEE802.11MAC protocol.It is proved that the performance improvement by cooperative communicationheavily depends on the selection of relay nodes. Most of previous work on relayselection only considers the factor of physical layer, for example, the channel quality ornode energy. However, in practice when there are multiple data flows in the network,the inter-node interference significantly affects network performance. Inappropriaterelay selection without considering inter-node interference may degrade networkperformance instead of improving it. To mitigate the impact of inter-node interference, this dissertation proposes a distributed interference-aware relay selection algorithm(DIRSA) from the view of the whole network that selects a relay node withconsideration of both inter-node interference and channel conditions. The algorithmreduces the inter-node interference in the network by effectively selecting relay nodes,thereby significantly reduces the collision probability and further improves the networkthroughput.In a single channel wireless network, the available spectrum is usually managed asone only channel resource, and when all nodes in the network contending the onlychannel will cause collisions and possible unfair competition. Multi-channel technologyis an important research direction to overcome this problem. Multiple channel accesstechnology allows neighbor nodes to transmit data simultaneously in different channels,which can increase the spatial reuse, improve the network performance and significantlyalleviate the problem of high collision probability and unfair access existing in singlechannel access networks. Existing multi-channel MAC protocols are mainly dividedinto two categories: the first is the fixed-width static multi-channel MAC protocol.Among these protocols, the channelization structure is pre-configured that the entireavailable spectrum is divided into sub-channels with equal channel width, which isdifficult to adapt to temporal disparity in the traffic demands of nodes. The second is avariable-width dynamic multi-channel MAC protocol. Unfortunately, most of theseprotocols are designed for the infrastructure wireless networks where a centralcontroller is responsible for variable-width channel allocations, which are not suitablefor the distributed ad hoc networks. Meanwhile, the sub-channel division is stillcoarse-grained that the channel width is chosen from several fixed values, e.g.5,10,20or40MHz. This dissertation presents a traffic-aware channelization MAC (TAC-MAC)protocol for the case that each node is equipped with a single half duplex transceiver.Specifically, the key features of TAC-MAC protocol are as follows. First, TAC-MACworks in a distributed manner. It adopts a window-based approach that each scheduleconsists of a negotiation window and a data window. And the node who firstly wins thecontention for access to the channel initiates a transmission schedule. There is no needof central controller for sub-channel division and channel width allocation. Second,TAC-MAC utilizes the whole available spectrum during negotiation windows, whichavoids the spectrum wastage generally existing in previous window-basedmulti-channel MAC. Furthermore, the size of the negotiation window can adapt to thenumber of contending nodes. Third, during data windows, beyond fixed-width orcoarse-grained channelization structure, TAC-MAC adopts a fine-grainedvariable-width channelization strategy based on orthogonal frequency divisionmultiplexing technique that can flexibly commensurate with the traffic demands of eachnode. Analysis and simulation results show that the TAC-MAC can significantlyimprove network throughput and reduce packet delay compared with both the fixed-width multi-channel MAC and the single channel802.11MAC protocols. And themore competitive the network condition is, the more advantage TAC-MAC will bring.