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A new modeling framework is introduced for the analytical study of medium access control (MAC) protocols operating in multihop wireless ad hoc networks, i.e., wireless networks characterized by the lack of any pre-existent infrastructure and where participating devices must cooperatively provide the basic functionalities that are common to any computer network. The proposed modeling framework focuses on the interactions between the physical (PHY) and MAC layers, and on the impact that each node has on the dynamics of every other node in the network. To account for the effects of both cross-layer interactions and the interference among all nodes, a novel linear model is introduced with which topology and PHY/MAC-layer aspects are naturally incorporated in what we define as interference matrices. A key feature of the model is that nodes can be modeled individually, i.e., it allows a per-node setup of many layer-specific parameters. Moreover, no spatial probability distribution or special arrangement of nodes is assumed; the model allows the computation of individual (per-node) performance metrics for any given network topology and radio channel model. To show the applicability of our modeling framework, we model wireless ad hoc networks that operate according to the IEEE 802.11 standard. To accomplish this, we present a comprehensive analytical modeling of the IEEE 802.11 and the derivation of many performance metrics of interest, such as delay, throughput, and energy consumption. Following recent advances in communication technologies, we also investigate the use of multiple antenna elements in both ends of the wireless link to conduct the first analytical modeling of wireless ad hoc networks that considers the impact of realistic antenna-gain patterns on network performance. As such, our modeling approach allows the study of ad hoc networks in which nodes are equipped with directional antennas, i.e., systems of antennas that are able to transmit/receive energy over intended directions. This modeling capability stands out from all previous analytical models, which have only dealt with omnidirectional or over-simplified antenna gain patterns, and which have not addressed the specific mechanisms of the MAC protocols used (e.g., backoff mechanisms). Lastly, we present the first analytical model for wireless ad hoc networks equipped with multiple-input multiple-output (MIMO) radios enabled with space-time coding (STC) that considers the impact of the underlying radio-based topology. In particular, we consider the space-time block coding (STBC) technique known as the "Alamouti scheme." We derive the effective signal-to-interference-plus-noise density ratio (SINK) of the Alamouti scheme under multiple access interference (MAI), and we propose the moment generating function (MGF) method to derive closed-form expressions for its symbol error probability under different modulation schemes and when fading paths are independent but not necessarily identically distributed. We apply the Alamouti scheme to IEEE 802.11 ad hoc networks with different antenna system configurations and compare their performance with respect to the basic single-input-single-output (SISO) system.