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Optically controlled reconfigurable antenna for 5G future broadband cellular communication networks

— This paper presents an optically controlled reconfigurable antenna for millimetre-wave frequency range. Silicon switches are used to control the optical reconfiguration, modifying the frequency response and radiation pattern of the antenna design. Therefore, the system can switch between the lightly licensed 28 GHz and 38 GHz frequency bands, useful for future mobile 5G broadband cellular communication networks. Experimental results with the reconfigurable antenna on 16-QAM and 32-QAM wireless transmission supported by photonic downconversion are successfully reported under 78 dB link budget requirement.


I. INTRODUCTION
The number of users of wireless technology has presented a continuous up growth during the last decades, pressing the development of new technologies to supply the market necessities. The dominance of smartphone in traffic web will bring on a saturation of the current network bandwidth capacity, which operates in frequencies lower than 2.7 GHz. Therefore, higher frequency bands have being investigated for high-capacity wireless systems, especially in unlicensed or lightly licensed millimeter wave (mm-wave) frequency bands, such as: the E-band (60-90GHz), W-band (75-110 GHz) and even sub-Terahertz (100-300GHz) [1][2][3]. To make available these efficient broadband wireless systems, operating in different frequencies, antenna is a key component to guarantee the applicability of the signal distribution. Although optical technologies can provide solutions to generate and distribute such signals, the electrical front-ends at these frequency regimes are still under development. Moreover, distributed antenna systems architectures have been shown a promising technology, due to the reduced wireless coverage in case of mm-wave links [4].
Optically controlled reconfigurable antenna for 5G future broadband cellular communication networks A potential band which has been proposed for future 5G broadband cellular communication networks and satisfies bandwidth requirements is 28 GHz and 38 GHz frequency bands [5]. In such mm-wave bands, unlike at 60 GHz and above, atmospheric absorption does significantly not contribute to additional path loss [6]. Moreover, the development of front-ends capable of operating in both bands simultaneously is necessary to support large-scale deployments. Radio over Fiber (RoF) is a potential technology to enable the integration of wireless and fiber optic networks by means of taking advantages of the benefits of optical fiber with the mobility and ubiquity of wireless networks [7][8]. It is based on the transmission of RF signals by using an optical carrier through optical fiber links. RoF is an essential technology for the provision of untethered access to broadband wireless communications in a range of applications including last mile solutions, extension of existing radio coverage and capacity, indoor and outdoor cellular systems, among others.
With respect of the exploration of the underutilized mm-wave frequency spectrum be for indoor and outdoor environments in Austin and New York cities [5]. Moreover, narrowband wideband measurements have been performed in diverse indoor and outdoor environments in order to enable the development of reliable prediction models for 60 GHz radio channels [10].
On the other hand, another interesting and promising solution for the recent technologies of wireless communications systems is the use of reconfigurable antennas [11]. This class of antennas enables to reconfigure not only the bandwidth, but also the radiation pattern and polarization pattern.
Reconfigurable antenna ensures flexibility design, once it presents frequency agile and makes use of software defined and cognitive radios to cope with extendable and reconfigurable multiservice, multistandard and multiband operation. Moreover, by using reconfiguration, an efficiently use of the spectrum and the power utilization is achieved. Our group has recently proposed the concept of an Adaptive and Cognitive Radio over Fiber (ACRoF) [12] based on the integration of an optically controlled reconfigurable antennas and RoF systems [13]. Using this new approach, it becomes possible to perform a high capacity optical backhaul with a broadband, energy-and spectrum-efficient wireless access segment, also exploiting new spectral resources.   In the current work, we propose to reconfigure its frequency response and radiation pattern by using a photoconductive switch illuminated via optical fiber by high-power 808 nm laser, as shown in Fig. 1. The photoconductive switch consists of a tiny piece of intrinsic silicon die (highlighted in the up right corner in Fig. 1), which can change its electrical properties when light is applied. In this way, its conductivity becomes reconfigurable as a function of the incident optical power. As soon as the photoconductive switch is illuminated, silicon semi-conductor changes for a conducting state by creating electron-hole pairs [19]. The incident photons must have enough energy to move electrons from the valence to the conduction band. Light in the near-infrared region is adequate for this process, since it strikes a balance between the absorption coefficient and the light penetration depth related to the light wavelength. In this way, it is possible to manage the switch conductivity by controlling the optical incident power. Silicon switch is strategically placed in one of the slots, making part of the antenna metallic body. In this work, the optical switch has been placed on the first and third slots.
Therefore, the reconfiguration of the radiation pattern and the frequency response is obtaining according to the power delivered by the fiber. When the laser is turned on the silicon switch starts to conduct, changing the electric size of the slot. Turning on just the switch placed on the first slot, the modification on the structure removes the high frequency resonance and the antenna operates on a single widest band, as shown in Fig. 3a. Illuminating the first and the third slot, simultaneously, it is possible to reconfigure the operation band, enhancing just the 38GHz frequency, as presented in Fig. 3b  Since the feeding structure excites a wide range of frequencies inside the waveguide structure, other operation bands can couple with the slots. Therefore, the distance between the slots for 28 GHz and 38 GHz generates a third operation band around 34 GHz. This band can also be useful for the 5G networks, but for this work, just 28GHz and 38GHz bands are the interest regions.   The constellation has been shown extremely poor for the "off" state of photoconductive switch and, consequently, the measured EVM RMS was 12.2%. The higher is the laser current; the better is EVM RMS , reaching 5.8% for 2.5 A. The measured constellation for "on" state is extremely better than the "off" state, as highlighted in the top-right inset of Fig. 7b. Additionally, SNR has also been significantly improved by 7 dB, as illustrated in Fig. 7a. These results demonstrate that the frequency-agile and optically steerable antenna properties can be efficiently applied to implement an adaptive wireless system in the mm-wave frequency range.

V. CONCLUSIONS
An optically controlled reconfigurable antenna for mm-wave applications was proposed, developed and implemented. Its frequency response and radiation pattern can be efficiently reconfigured by using a silicon photoconductive switch. It has been experimentally investigated by transmitting a 40 Mbaud 16-QAM and 32-QAM wireless system at 38 GHz and 39 GHz, respectively, under 78 dB link-budget requirements. Increasing the optical power on the photoconductive switch, the measured EVM RMS has been gradually enhanced from 12.2 % to 5.8 %. Furthermore, the extremely poor constellation for "off" state becomes better as the switch was turned to "on" state. Moreover, the SNR also increased from 14.15 dB to 22.3dB, respectively, for "off" state to "on" state. The promising results encourage us to continue researching towards systems supporting unexploited frequency bands beyond the current commercial systems.