A circular phased array antenna that can generate orbital angular momentum (OAM) radio beams in the 10 GHz band is described. The antenna consists of eight inset-fed patch elements and a microstrip corporate feeding network. A full-wave electromagnetic simulator is used to aid the antenna design and theoretical simulations are confirmed by measurements.Introduction: Recently, orbital angular momentum (OAM) for radio communications has attracted much attention as it allows the possibility of transmitting multiple signals simultaneously at the same frequency, which may be used to increase channel capacity and potentially improve spectrum efficiency. The OAM characteristic was initially investigated and gave rise to many applications in the optical region [1,2]. In [3], an optical wireless link reached a data rate of 2.56 Tbit/s by using OAM beams. Recently, OAM was proposed for radio communications at lower frequencies [4,5]. The first laboratory experiments of OAM radio beam generation and detection were presented in [6]. In 2012, an OAM-based radio was demonstrated experimentally in a realworld setting, with two radio waves encoded with different OAM states and operating at the same frequency over a distance of 442 m [7].There are several methods that have been investigated for the generation of OAM radio beams. In [6], a discrete eight-step staircase nonfocusing phase reflector was used, where each staircase of the reflector introduced a discontinuous phase step of 2π/8 radians. The beam of OAM mode +1 was generated when a plane wave was reflected from the spiral reflector. Using a similar approach, a helicoidal parabolic antenna was fabricated and tested in [7]. Circular phased array antennas were predicted to generate OAM beams in [4,5,[8][9][10], and a timeswitched array was recently suggested to be an alternative option in [11]. For an N-element OAM circular phased array, all radiation elements are fed by the same signal but with an incremental phase shift. This inter-element phase shift can be calculated by 2πl/N, where the integer l is the OAM mode number, such that the phase will be incremented by 2πl radians in one geometrical rotation around the array axis. Hence, a 'twisted' radio beam at OAM mode l can be generated.To the authors' best knowledge, all previous published research relating to OAM generation by circular phased arrays has only considered numerical predictions. Hence, in this Letter, we present a practical 8-element circular phased array designed for OAM-based communications. The antenna structure, including the feeding network, is investigated based on both numerical and experimental methods, and the generation of an OAM radio beam is confirmed.

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A means of encoding and decoding data using wireless orbital angular momentum (OAM) modes is proposed and analysed. Source data symbols are used to select an OAM mode, which is generated using an 8-element circular array. A 2-element array is used to detect the mode by estimating the phase gradient of the received signal, and hence identifying the transmitted data symbol. The results are presented in terms of mode estimation error.

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