Cooperative Cross-Layer MAC Protocols For Wireless Ad Hoc Networks

Wireless ad hoc networks are dynamically self-organized and self-configured, and do not need any infrastructure, which render wireless ad hoc networks quite suitable for the places constrained by economic condition and geographic location. The future communication systems are required to offer not only large coverage and reliable transmission but also high transmission rate and energy efficiency.However, the rapid development of wireless communication systems is seriously restricted by the hostile transmission environment and limited spectrum resources. Due to the sharing nature of wireless channels and the lack of central controller in the networks, each distributed node has to contend randomly for the wireless channel. When the node density is quite high, frequent collisions may significantly reduce the utilization rate of wireless resources. Accordingly, IEEE 802.11 DCF was proposed to coordinate the distributed nodes occupying the wireless channels in an orderly fashion, which can effectively reduce collisions. Although simple, IEEE 802.11 DCF still has its own drawbacks, such as low spatial reuse and low energy efficiency.Aiming at addressing these limitations, several physical layer techniques were introduced to improve system performance:transmission power control scheme was used to enhance energy efficiency; interference model was employed to increase spatial reuse; rate adaptation scheme was introduced to improve throughput. However, these traditional MAC protocols cannot improve the system performance effectively when the direct link quality is poor. Fortunately, cooperative MAC protocols were proposed to coordinate the distributed nodes to form virtual antenna array, which can be used to mitigate the detrimental effects of wireless channel; thus, data rate and energy efficiency can be significantly improved. Most of the existing cooperative MAC protocols considered one-hop performance only, however, end-to-end multi-hop performance in wireless ad hoc networks was not taken into consideration.By fully utilizing the wireless broadcast advantage and wireless cooperative advantage, we introduce novel cooperative communication model and cross-layer design approach, which can significantly enhance the system throughput and energy efficiency, as well as keep the protocol complexity in a reasonably low level. In general, the main research discussed in this thesis can be described as follows:1) Under the constraint of single channel and single transceiver, a new joint scheduling and transmission power control MAC protocol is proposed, in which scheduling mechanism is adopted to eliminate strong level of interference while transmission power control scheme is implemented to limit the level of interference. Moreover, in order to activate more concurrent data transmissions, when calculating the schedulable set, we activate the far nodes preferentially and redistribute the maximum tolerable interference. Besides, several unnecessary transmissions of data packets and acknowledgement packets are cancelled in advance, which can further enhance energy efficiency.2) Through combining distributed space-time coding scheme and adaptive modulation scheme at the physical layer and employing truncated ARQ scheme at the data link layer, we propose and evaluate a novel cooperative cross-layer MAC protocol for wireless multi-hop networks. By fully utilizing the wireless broadcast advantage and wireless cooperative advantage, a distributed two-transmitter two-receiver model for cooperative communication is constructed and cooperative diversity can be achieved in only one phase. Under different system models with or without the truncated ARQ scheme, performances of the average PER and SER with different modulation types over Rayleigh fading channel are analyzed.3) We propose a novel link-utility-based cooperative MAC protocol for wireless multi-hop networks. Through sharing distributed antennas and forming virtual antenna array, transmitter and its partner can finish the cooperative transmission in only one phase. Based on their specific transmission type and the instantaneous channel measurements, each node tries to maximize its own link-utility by jointly adjusting transmission rate and power. Moreover, in order to select the cooperative node, we also propose a distributed back-off scheme, which can not only select the best node efficiently, but also avoid the packet collisions effectively.In sum, aiming at addressing the problems in the existed MAC protocols, cooperative communications and cross-layer design method are proposed to improve the throughput and energy efficiency performance in wireless multi-hop ad hoc networks, meanwhile the protocol complexity is also kept in a reasonably low level. Finally, problems to be solved in this field and future research topics are summarized.