Narrowband Satellite Communications: Challenges and Emerging Solutions

52 INTRODUCTION In the context of satellite communications (SATCOM), the term narrowband (NB) generally refers to the ultra-high-frequency (UHF)-, L-, and S-bands, where available bandwidth is limited compared to the higher-frequency bands. These lower-frequency bands in the 300-MHz to 4-GHz range are attractive to the DoD because of their better penetration into buildings and through foliage and their reduced signal attenuation in adverse weather conditions. Because of limited available bandwidth, primary applications of NB-SATCOM are voice and low-rate (e.g., 64 kb/s) data transfer. The existing legacy DoD NB-SATCOM constellation consists of the UHF Follow-On (UFO) and Fleet Satellite systems operating in the 240to 320-MHz range. Each UFO satellite supports seventeen 25-kHz channels and twenty-one 5-kHz channels.1 This legacy constellation is currently approaching the end of its life; hence, it is being replaced by the Mobile User Objective System (MUOS), which is the DoD’s next-generation NBSATCOM system.2 MUOS operates at 300–320 MHz for the uplink and 360–380 MHz for the downlink. It consists of four operational satellites in geosynchronous Earth orbit (GEO) plus one in-orbit spare. When fully operational, MUOS is expected to provide 39.2 Mb/s of global capacity to terminals compatible with the new Wideband Code Division Multiple Access (WCDMA) waveform.3 Each MUOS satellite also includes a legacy payload to support the legacy UHF terminals. The DoD also uses leased services from commercial SATCOM providers such as Iridium and Inmarsat. Although MUOS and leased commercial SATCOM services are expected to provide orders of magnitude more capacity compared to the legacy UHF system, emerging requirements still present significant challenges. These challenges include developing low-cost, low-power, small-footprint terminals; continuing support for legacy SATCOM terminals; achieving resilience against kinetic and electronic attacks; and increasing the efficiency of ABSTRACT