Time Division Multiple Access And Cross-layer Design In Ad Hoc Networks

With the development of the wireless communication technologies and the increasing requirement of communication at any time and any where, ad hoc networks have gained a lot of attention for their unique characteristics, and have become one of the most important trends of the mobile communication. The media access control sub-layer (MAC) controls how the nodes access the channel, and influences the performance of ad hoc networks a lot. Special MAC protocols are required to adapt to the characteristic of the ad hoc wireless channel. Application in military networks is one of the most important application scenarios and the main impetus to the development of ad hoc networks. Adopting TDMA(Time Division Multiple Access) to the MAC layer can transmit real-time applications and fight against the interferences in military Ad hoc networks. It is challenging to apply TDMA to ad hoc networks because they are distributed, self-organized, multi-hops and topology changeable. Two problems have to be solved: slot synchronization and slot assignment.The two key problems of slot synchronization and slot assignment are studied when applying TDMA to military ad hoc networks.To optimize the system performance, the parameters from routing layer and physical layer (PHY) are adopted to the design of MAC, which also implements the basic idea of cross-layer design.Slot synchronization of network nodes is the prerequisite of realizing TDMA and will influence the network performance directly. A slot synchronization scheme based on the internal parameters analysis is proposed to meet the particular requirements of military ad hoc networks. The scheme is made of two parts: the time synchronization protocol and the setting of guard interval. The time synchronization protocol can realize the coarse slot synchronization. And guard interval is inserted between two neighboring slots to allow the jitter of slots, which can avoid collisions. Corresponding to the use of frequency hopping(FH) technology in PHY, a slot synchronization scheme based on FH synchronization is proposed. In the scheme, each node in the network uses a slot counter to position slots. By compensating the time when the separator is detected with the estimated delays, the receiver's slot counter can synchronize to the sender's. Because the proposed scheme can avoid transmitting concrete time information in the network, the channel utilization can be improved. The proposed scheme has been used in military communication devices and shows good performance.Slot assignment is the primary part of TDMA. Dynamic slot assignment is required toutilize the limited wireless bandwidth efficiently. In order to adapt to the variable topology and ensuring the channel access, an on-demand and dynamic slot assignment protocol (ODSA) is proposed. ODSA assigns a slot to a node based on the information in its two-hop neighborhoods, and realizes the slot application in the selected slot, which can decrease and solve slot application collisions efficiently. Slots are assigned to nodes when needed, and will be released when they are no more used. Therefore, ODSA can adapt to the variations of topology and application. The assigned slots will be kept by the nodes until applications or topologies are changed. So, the channel access of nodes with slots can be ensured. To adapt to the variation of nodes number in the two-hop neighborhood, a binary-tree-based adaptive slot assignment protocol (BASAP) is proposed. In BASAP, the time frame structure is corresponded to a binary tree structure. It is very convenient to manage the slot assignment information and select a slot with a certain criterion in the binary tree structure. The performance of BASAP is much better than that of ODSA due to its changeable frame length.Cross-layer design is the trend of ad hoc protocol design. The whole network performance can be improved by coordinating parameters of different layers. Therefore, influence from other layers is adopted to the design of TDMA MAC. A combined design of MAC and routing to provide QoS (CMRQ) is proposed. In CMRQ, the control packets from the routing protocol are embedded into that of the MAC protocol. The QoS requirement on bandwidth of an application can correspond to a certain frame length, so the QoS can be ensured when all the nodes on the found route have owned slots with the frame length corresponding to the QoS requirement. Adopting the information from PHY to the design of MAC protocols is highlighted recently. With the existence of channel fluctuation, different users have different instantaneous channel gains, which can be used to realize multi-user diversity. A TDMA scheme to utilize multi-user diversity is proposed for ad hoc networks. In the scheme, a slot is divided into control stage and data stage. In the control stage, nodes gather the link information and solve link access collisions; in the data stage, the nodes which have access successfully transmit their data packets. The proposed scheme can utilize the multi-user diversity efficiently to improve the network throughput.