How to evaluate SOTM antennas, modems, and architectures

The two key components that make up a “Satellite on the Move” (SOTM) terminal are the antenna¹ and the modem. SOTM terminals can be combined to form a mobile network managed by at least one controller, typically a stationary terminal with a large aperture antenna. SOTM networks operating in the traditional commercial satellite Ku-band (14-14.5 GHz uplink) have very unique operating constraints that must be considered before acquiring and operating such a system. These SOTM operating constraints must be analyzed to determine capability and performance: 1. Regional EIRP Spectral Density (ESD) limits 2. Antenna pointing accuracy and allowable maximum transmit ESD 3. Satellite transponder performance 4. Modem/Link bit error rate (BER) 5. SOTM and Controller earth terminal performance This paper will address these system constraints in developing a concept of “Available” vs. “Achievable” operating modes of a SOTM system. The SOTM terminal theoretical “Achievable” operating mode; measured in terms of data rate (bps) and spectral efficiency (bps/Hz) is determined by its EIRP, maximum allowable transmit ESD and modem´s operating E_b/N₀. The practical SOTM operating modes are established by EIRP, ESD, and the “Available” waveforms provided by the modem. Through equations, analysis, and examples; this paper will show that the best performing SOTM system, in terms of throughput, spectral efficiencies, connectivity, and having the greatest worldwide flexibility, will be a system that matches the achievable modes to its available operating modes as closely as possible. This paper will also cover “Link Availability” and “Link Margin” that are traditionally used to address weather/channel impairment and their overall impact to SOTM network throughput as well as introducing a practical mitigation technique. The final part of this paper wi- l show that direct peer-to-peer (PtP) SOTM connectivity, herein simply referred to as PtP-SOTM, does not always have the lowest latency for data transfers. Lower latency can be achieved through the use of a large aperture antenna serving as a data relay between two SOTM nodes. SOTM architectures can be developed to trade data rate (DR), link spectral efficiency and data transfer delay (DTD).