UHF adjacent channel SATCOM interference: Present, interim and future

Efficient utilization of military UHF 5 kHz and 25 kHz satellite communication (SATCOM) channels requires the selection of data rates and modulation waveforms that maximize the allocated channel bandwidth without causing significant interference to adjacent channel users. The Joint Chiefs of Staff (JCS) and the Joint Interoperability Engineering Organization (JIEO) have directed new modulation waveforms and rates. The optimum selection of modes and rates is also influenced by the requirement for interoperability with fielded equipment. Examination of the spectral requirements for SATCOM operation is divided between uplink and downlink concerns. Attention is focused on the effects of channel spacing and waveform shaping for controlling uplink adjacent channel interference as military standards are applied and the UHF Follow-On (UFO) satellite constellation appears on orbit. The required adjacent channel emission control of the uplink signal is examined for both DAMA and non-DAMA applications. Before the advent of MIL-STD-188 interoperability standards, downlink adjacent channel interference rejection was never included in equipment specifications for SATCOM modems or transceivers. The test waveforms for measuring downlink interference rejection are examined and suggestions are made for future improvements in military SATCOM interoperability standards. If performance specifications do not sufficiently match real-world conditions, frustrated SATCOM radio users will blame intermittent interference on link margin, when it might be equipment limitations. The special case of the low-gain, airborne hemispherical-coverage antenna user is examined in detail for 25 kHz SATCOM operation to show that waveform shaping is required. Finally, the performance of the MD-1269A 70 MHz multipurpose modem and the RT-1273AG 100 watt multimission UHF SATCOM transceiver, designed for uplink spectral containment and downlink interference rejection, are both shown to meet the new SATCOM performance standards