Simultaneous operation of UHF communication channels on an airborne platform

Ultra high frequency (UHF) channels provide a reliable medium for conveying voice and data information between users within a communications network. UHF communication channels may be categorized as either Line-of-Sight (LOS) or Satellite Communication (SATCOM) channels. UHF LOS and SATCOM systems are typically composed of voice/data sources, cryptographic devices (for secure channels), data terminal sets (modulator/demodulator/satellite link controller), UHF radios, and antennas. In designing a UHF communications system, the systems engineer must characterize the equipment, the physical channel, and the satellite transponder to determine if the information can be successfully transferred between users. Published text describing the techniques for characterizing and analyzing UHF communication systems and channels in single-channel applications exists and will not be covered in this paper. However, setting up successful links in an environment where several users are operating over multiple UHF channels simultaneously utilizing several UHF antennas, referred to as Simultaneous Operation (SIMOP), presents a more challenging problem, even for the experienced systems engineer. This paper describes problems and proposed solutions for UHF SIMOP in an airborne platform. Since baseband equipment for individual communication networks have minimal adverse effect on SIMOP performance, this paper focuses on the radio and antenna equipment. The airborne platform is unique in that it uses low-drag, few-gain, closely-spaced antennas which lead to low isolation. Concepts such as adjacent-channel interference, intermodulation distortion, broadband transmitter noise, harmonic and spurious emissions, receiver desensitization and cross modulation are described as they apply to SIMOP applications. Budget analysis using typical radio specifications shows that the predominant adverse effects are driven by transmitter broadband noise and intermodulation. Solutions to SIMOP problems, including antenna isolation, filtering, frequency management, and signal cancellation are addressed, with the goal that the techniques discussed may be applied successfully to existing or future UHF communications systems with SIMOP requirements.<
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