Cross-layer mitigation techniques for channel impairments

Next-generation communications satellite constellations will use advanced radio frequency (RF) technologies to provide Internet protocol (IP) packet-switched high-speed backbone transport services for various user communication applications, with ever-increasing traffic demand. These applications range from data services to imagery, voice, video, and other potential emerging applications. Satellite uplinks and downlinks may endure channel impairments that have fades of varying durations due to weather, communications on the move (COTM) blockages, scintillation, terrestrial multipath, or jamming. Satellite payloads and ground terminals must be able to mitigate this wide range of impairments and optimize the use of available spectrum to deliver the highest possible data rates while maintaining a required quality of service (QoS). A suite of mitigation techniques-including channel interleaving and forward error correction (FEC) in the physical layer, dynamic coding and modulation (DCM) and automatic repeat request (ARQ) in the data link layer, and application codec adaptation (ACA) in the application layer-has been proposed for various channel fades. Since each mitigation strategy could potentially interact with another, it is essential not only to assess the performance of each mitigation technique, but also to understand how multiple cross-layer techniques work together. This paper describes an emulation study of channel impairment mitigation using a combination of dynamic modulation (DM), ARQ, and ACA for various channel fades. A real-time emulation test bed was established by integrating satellite-to-terminal real-time Ethernet configurable hardware (STRETCH) and satellite link emulator (SALEM) test beds, both of which are unique capabilities developed in-house at The Aerospace Corporation. STRETCH provides modulation/demodulation, coding, interleaving, and various types of channel fading. SALEM implements a range of mitigation techniques, such as ARQ, DM, and ACA. ARQ retransmissions are triggered in the absence of acknowledgment, DM is invoked upon SNR changes, and ACA is called when the available data rate changes. Results show that DM and ACA successfully mitigate channel fades of longer durations. Faster fades with fluctuating channel gains but a steady SNR average over a given time window do not trigger DM, but endure bit errors and packet drops caused by instantaneous low SNR values. ARQ retransmissions successfully mitigate these types of channel fades. This paper presents descriptions of the test bed architecture, mitigation techniques, test scenarios, and test results.