# The Recent Advancement in Unmanned Aerial Vehicle Tracking Antenna: A Review
## 1 Introduction  
During the last two decades, unmanned aerial vehicle (UAV) has been deployed to perform high risky operations where deploying humans presents great danger to their health notably in fire-fighting [1, 2], chemical spraying [3, 4, 5], search and rescue missions [5, 6], disaster network communication [7, 8] and surveillance operations [9]. Undoubtedly, UAV will be pivotal in medical and blood delivery for troops in the frontline when it is dangerous to send in medical personnel. The market of UAV has a positive outlook, as could be seen from Figure 1. The major drivers of the UAV market are the remotely operated commercial UAV and the emergency management sector. Particular interest is the remotely operated commercial UAV where the channel model ranges from direct line of sight (LoS) to non-LoS propagation communication [10]. UAVs are deployed in urban, semi-rural,and rural environments which are fully captured using LoS and non-LoS scenario. Each environment characterized by different link obstacle topology. For instance, in the rural area where there is less obstacle, the channel model conforms to the free space path loss model suitable for LoS communication. Different from the NLoS model, the UAV ground control station (GCS) deploys the LoS model to focus the antenna beam directly to the airborne UAV using a highly directional antenna rather than an omni-directional antenna to maximize the received signal power. Furthermore, the assigned UAV transmit powers which varies might become inadequate as communication range increases. For instance, wifi operated UAV is limited to 20 dBm while the LTE operated UAV is limited to 24 dBm in uplink and 43-48 dBm in downlink) [10]. Hence, there is a need to rely on tracking antenna to deliver the needed signals over a wider communication range in unpredictable wireless communications link. Obviously, using highly directional antenna enhances the received signal strength (RSS) of the airborne platform. Thus, there is a need to reduce co-channel and cross channel interferences. Additionally, the need for automatic repeat request (ARQ) as well as hybrid automatic repeat request (HARQ) in the face of loss of signal when deployed in autonomous fashion are also reduced. Moreover, the aerial platform records a higher signal-to-interference-noise ratio (SINR) when a packet is successfully delivered. From the perspective of energy efficiency and network lifetime, deploying omni-directional antenna connotes the use of more energy to achieve a targeted reliable communication link between the GCS and the air-borne platforms. Intuitively, directional antenna is highly desirable for UAV communication because it improves pointing accuracy, and minimizes the antenna sidelobes and antenna beamwidth. In high-frequency band communication, sustaining precision communication between the ground station and the airborne platform requires significant improved pointing accuracy because of the short wavelength and range [11; 12]. Meanwhile, using a directional antenna is limited by azimuth and elevation angles, and must always be directed to the target [14]. It is important to note that optimal directional antenna should consider UAV look angles (yaw, pitch, and roll) which are dynamic as they are motion-based. Therefore, the UAV tracking antenna must overcome this limitation by constantly adjusting the beam target direction. Keeping this in mind, different researchers have explored several ways to address this problem and have discussed in this article. The major contributions of this work are listed below:  
Figure 1: UAV market outlook [13]. * This work provides an exhaustive review on the state-of-the-art UAV antenna tracking technology as well some discussion on tracking antenna fundamentals. * Instead of relying on the electro-mechanical UAV antenna tracking technology, this work proposes the adoption of MIMO beam-forming technology as an alternative to the electro-mechanical UAV antenna tracking.