Robot swarms, which constitute typical multiuser systems, have played more and more important roles in many areas. In this paper, a directional modulation(DM) scheme for frequency diverse array(FDA) based on the leakage power minimization criterion is proposed, which can be used in a robot swarm system as well as other multiuser scenarios. Based on this scheme, location-dependent secure transmission can be obtained in the multiuser downlink channels under certain circumstances, where independent data streams can be sent to different legitimate users safely. Meanwhile, the difficulty for perfect eavesdropping is significantly increased compared with phased-array-based DM. We also use the block successive upper-bound minimization algorithm to optimize the frequency offsets of the array, and the algorithm can be effectively performed. This scheme utilizes the baseband model of the FDA, which is another contribution of this paper. It solves the problems brought by the time-varying nature of the FDA and reveals the distance-angle correlation of the FDA more fundamentally. Based on that, besides the proposed FDA-DM scheme, a novel multiple access technique, named as link division multiple access, is also preliminarily proposed. It allows the transmitter to distinguish multiple users in the same or proximal locations without the help of time division, frequency division, or code division. At last, numerical results are presented to verify the superiorities of LDMA and the proposed FDA-DM scheme.

Unmanned aerial vehicle (UAV) systems undergo a period of rapid development in both civil and military scenarios. A major challenge in the malicious jamming environment is to guarantee the reliability of UAV communications links. Frequency hopping (FH) is one of the most commonly used means of combatting the influence brought about by jamming. In this paper, we integrate low-rate codes into an anti-jamming FH communications system, and propose an efficient and low-complexity turbo-Hadamard code scheme. Tail-biting is applied to design the component convolutional-Hadamard codes, and a corresponding decode algorithm is used for implementation in the UAV hardware platform. Numerical simulation results demonstrate that the anti-jamming performance of this method is improved as compared with conventional concatenated codes. Finally, we compare the complexity and transmission efficiency of the proposed algorithm with the algorithms implemented on the field programmable gate array (FPGA) platform in detail.

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