  The future upgrade of Keck II telescope's adaptive optics system will include
a pyramid wavefront sensor working in the near-infrared (J and H band). It will
benefit from the recently developed avalanche photodiode arrays, specifically
the SAPHIRA (Selex) array, which provides a low noise ($<$ 1 e- at high frame
rates). The system will either work with a natural guide star (NGS) in a single
conjugated adaptive optics system, or in a laser guide star (LGS) mode. In this
case, the pyramid would be used as a low-order sensor only. We report on a
study of the pyramid sensor's performance via end-to-end simulations, applied
to Keck's specific case. We present the expected Strehl ratio with optimized
configurations in NGS mode, and the expected residual on low orders in LGS
mode. In the latter case, we also compare the pyramid to LIFT, a focal-plane
sensor, demonstrating the ability of LIFT to provide a gain of about 2
magnitudes for low-order sensing.
