The Fundamental Multi-Baseline Mode-Mixing Foreground in 21 cm EoR Observations

The primary challenge for experiments measuring the neutral hydrogen power spectrum from the Epoch of Reionization (EoR) are mode-mixing effects where foregrounds from very bright astrophysical sources interact with the instrument to contaminate the EoR signal. In this paper we identify a new type of mode-mixing that occurs when measurements from non-identical baselines are combined for increased power spectrum sensitivity. This multi-baseline effect dominates the mode-mixing power in our simulations and can contaminate the EoR window, an area in Fourier space previously identified to be relatively free of foreground power.