I. Introduction
An Unmanned Airborne System (UAS) mainly has two basic types of communication links, namely control and non payload communication link (CNPC-link) and data-link. CNPC links are related to safe operation of UAVs. Highly reliable, low-latency and secure two-way communications, usually with low data requirement, are needed in CNPC link. Data-link is to support mission-related communications. UAV-based ubiquitous coverage, UAV-based relaying and UAV-based information dissemination and data collection fall under data link. The data link usually operates in HF, VHF, UHF, L, S, C and K band of frequencies, and the choice is based on application. High Data rate and high reliability are required in data link and high reliability is required in CNPC link.