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Utilizing smart/directional antennas in MANETs (mobile ad hoc networks) have been widely studied in recent years. Since they offer longer transmission ranges than omnidirectional antennas, decreased number of hops is needed for packet transmissions, and at the same time they cause interference only to a specific direction, which enables simultaneous transmissions between neighboring nodes in the network. Therefore the network capacity is significantly improved. The preceding features promote the utilization of smart/directional antennas especially in military communications, where enhanced capacity and quality of service are desired. However, before exploiting the benefits of smart antennas, development of higher layer protocols is a necessity. In this paper we are proposing a novel, TDMA based MAC protocol, termed as TDMA-SB (TDMA slot borrowing scheme). The proposed protocol utilizes smart antennas and location awareness of the nodes provided by GNSS (global navigation satellite system). It also introduces a novel slot borrowing scheme to utilize the unused time slots in a TDMA frame. The performance of the proposed protocol is evaluated in a computer based simulations performed with OPNET network simulator. Based on the simulation results it can be concluded, that the proposed TDMA-SB protocol is well-suited for MANETs, and from the three simulated protocols it is also the best choice, when collision free data transmissions together with maximized throughput are essential.