Dynamic Binary Channel Delay Emulation with Picosecond-Scale Precision

Abstract

There is growing interest in optical communications for satellite crosslinks. These crosslinks can be simultaneously used for data exchange, time synchronization, and even position determination. Use cases range from dense constellations of small satellites flying in formation to deep space links. It is difficult to emulate such channels under laboratory conditions because the range between any two satellites is both large and constantly changing. Channel emulators for cis-lunar space, for example, must provide delays for link ranges up ~1.5 light-seconds that change by many kilometers per second. Both requirements far exceed the capabilities of off-the-shelf solutions. This paper describes an FPGA-based binary channel emulator that applies a dynamic delay and supports a variety of noncoherent free-space optical (FSO) waveforms. The delay is applied at the physical layer, in contrast with packet-level delays implemented by network emulators. We present two approaches: one based on blind oversampling designed to work with any high-rate transceiver, and another using specific features of the transceivers in Xilinx FPGAs to allow delay adjustment in single-picosecond increments. A detailed implementation is described, addressing issues from initial digitization, accurate delay calibration, and dealing with enormous memory bandwidth. The delay emulator is entirely electronic but can be integrated with other equipment for end-to-end optical testing. Using Ethernet (SGMII) as a placeholder for the over-the-air waveform, the prototype demonstrates relay of gigabit user traffic interleaved with Precision Time Protocol (PTP) messages that are used to measure the channel delay in real-time. Future efforts will include support for coherent optical waveforms.