  We describe the validation of the HERA Phase I software pipeline by a series
of modular tests, building up to an end-to-end simulation. The philosophy of
this approach is to validate the software and algorithms used in the Phase I
upper limit analysis on wholly synthetic data satisfying the assumptions of
that analysis, not addressing whether the actual data meet these assumptions.
We discuss the organization of this validation approach, the specific modular
tests performed, and the construction of the end-to-end simulations. We
explicitly discuss the limitations in scope of the current simulation effort.
With mock visibility data generated from a known analytic power spectrum and a
wide range of realistic instrumental effects and foregrounds, we demonstrate
that the current pipeline produces power spectrum estimates that are consistent
with known analytic inputs to within thermal noise levels (at the 2 sigma
level) for k > 0.2 h/Mpc for both bands and fields considered. Our input
spectrum is intentionally amplified to enable a strong `detection' at k ~0.2
h/Mpc -- at the level of ~25 sigma -- with foregrounds dominating on larger
scales, and thermal noise dominating at smaller scales. Our pipeline is able to
detect this amplified input signal after suppressing foregrounds with a dynamic
range (foreground to noise ratio) of > 10^7. Our validation test suite
uncovered several sources of scale-independent signal loss throughout the
pipeline, whose amplitude is well-characterized and accounted for in the final
estimates. We conclude with a discussion of the steps required for the next
round of data analysis.
