# The Recent Advancement in Unmanned Aerial Vehicle Tracking Antenna: A Review
## 5 Multiple Input Multiple Output Technology
### Limitations of Deploying MIMO for UAV Tracking  
MIMO beam-forming techniques can adequately replace the mechanical beam-steering technique of antenna UAV tracking. However, before the full adoption of the technique, the following issues affecting MIMO technique must be addressed.  
**(a)**: _Pilot Contamination:_ Pilot contamination is a scheme in MIMO technology in which the pilots or preambles are transmitted by the transmitter to acquire the channel state information for the determination of the beam weights [153; 154; 155]. The issue is very crucial and critical, and, as such, many researchers have proposed various strategies to address the issue. Reference [156] proposed the use of transmit power control schemes; reference [157] suggested the use of graph coloring scheme, in which difference nodes are assigned difference colors to mitigate against interference; and reference [158] studied the downlink capacity under massive MIMO pilot contamination.
**(b)**: _MIMO Rank Deficiency:_ MIMO technology thrives under the assumption that the various multipath channels paths can be fully decoupled. Therefore, resulting to a full rank [159; 160]. This implies that the channels are not carrying redundant information and instead of the signals to be uncorrelated, they are related. Rank deficiency is usually experienced in MIMO technology when there are insufficient scatterers in the environment which leads to modelling the channel as a Rician probability density function (PDF). This is instead of the popular Rayleigh fading channel with Rayleigh PDF with lots of delayed path. This issue can be addressed by introducing artificial scatters in the signal path to induce NLoS scenario.
**(c)**: _MIMO Complexity:_ The computational complexity of MIMO technology as we migrate from the domain \"Non- Massive MIMO\" to \" Massive MIMO\" regimes. In massive MIMO, the antenna array cardinality increases in contrast to non-massive MIMO configuration. It is not surprising that massive MIMO can support numerous users in the context of MIMO multiuser (MIMO-MU). Using this technique, a GCS equipped with MIMO-MU can adequately support and control many UAVs, simultaneously. In other to address this issues, several MIMO complexity models have been proposed [161; 162; 163]. Most of the proposed strategies are centered on optimal antenna selection criteria.
(UPA) to track the UAV in all the sectors. The support for MIMO-BF lies in the fact that it can exploit the instantaneous channel attributed to align the beam weight and, hence, is reconfigurable. Secondly, a different modulation scheme could be adopted to respond to the varying channel state information, Thirdly, by introducing a delay circuit, it can alter the time and frequency components of the signal and hence, track the UAV dynamic flight parameters. By changing the antenna fed with the delay, the beam could rotate in four 90 degree steps. Beam-forming antenna can be etched using microstrip material because of the desirable form factor which can be easily fitted into the UAV GCS with easy. Furthermore, slot antenna configuration has interesting possibilities as well. The active sector can be electronically switched based on the RSSI or the beam weights which are computed by the GCS module. From the context of antenna polarization, circular polarization will be ideal in this fashion when compared to horizontal and vertical polarization. Circular polarization will ensure that there is no polarization mismatch, which will be result in the loss of signal.