Currently, unmanned aerial vehicles (UAV) have been widely used in many applications, such as in transportation logistics, public safety, or even in non-terrestrial networks (NTN). In all these scenarios, it is an important issue to model channel behavior between the UAV and the user equipment (UE) on the ground. Among these channel features, a critical parameter that dominates channel behavior is the probability of the line-of-sight (LOS), since the statistical property of the channel fading can be either Ricean or Rayleigh, depending on the existence of LOS. Besides, with knowledge of LOS probability, operators can design approaches or schemes to maximum system performance, such as the serving coverage, received signal to noise ratio (SNR), or the bit error rate (BER) with the limited transmitted power. However, the LOS UAV channel is likely difficult to acquire or derive, as it depends on the deployment scenario, such as an urban or rural area. In this paper, we generated four different scenarios defined by the ITU via the ray tracing simulator. Then, we used the spatial geometric relation and the curve fitting approach to derive the analytic models to predict the probability of the UAV LOS channels for different scenarios. Results show that our proposed relationships yield better prediction results than the methods in the literature. Besides, an example of establishing UAV self-awareness ability for the deployed environment via using proposed models is also provided in this paper.

Iterative block decision feedback equalization with hard-decision feedback (HD-IBDFE) was proposed for single-carrier transmission with frequency-domain equalization (SC-FDE). The detection performance hinges upon not only error propagation, but also the accuracy of estimating the parameters used to re-compute the equalizer coefficients at each iteration. In this paper, we use the erasure zone (EZ) to de-emphasize the feedback values when the hard decisions are not reliable. EZ use also enables a more accurate, and yet computationally more efficient, parameter estimation method than HD-IBDFE. We show that the resulting equalizer coefficients share the same mathematical form as that of the HD-IBDFE, thereby preserving the merit of not requiring matrix inverse operations in calculating the equalizer coefficients. Simulations show that, by using the EZ and the proposed parameter estimation method, a significant performance improvement over the conventional HD-IBDFE can be achieved, but with lower complexity.

In order to satisfy the user's demands for faster data rates and higher channel capacity, fifth generation (5G) wireless networks operate in the frequency at both sub-6GHz and millimeter wave bands for more abundant spectrum resources. Compared with the sub-6G bands, signals transmitted in the millimeter bands suffer from severe channel attenuation. A drone small cell (DSC) has been proposed recently to provide services outdoors. Not only does DSC have high maneuverability, it can also be deployed quickly in the required regions. Therefore, it is an important issue to establish the Air-to-Ground (ATG) channel model by taking into account the effects of building shielding and excess loss in various DSC deployments at different frequency bands. In this paper, we synthesize the ATG channels of the DSC and approximate the excess path loss of the ATG for different urban environments based on the ITU-R standard. With the approximated curve fitting relations, the proper height of the drone base station that satisfies a certain connected probability can be easily obtained for different scenarios.