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A critical challenge in measuring the power spectrum of 21cm emission fromcosmic reionization is compensating for the frequency dependence of aninterferometer's sampling pattern, which can cause smooth-spectrum foregroundsto appear unsmooth and degrade the separation between foregrounds and thetarget signal. In this paper, we present an approach to foreground removal thatexplicitly accounts for this frequency dependence. We apply the delaytransformation introduced in Parsons & Backer (2009) to each baseline of aninterferometer to concentrate smooth-spectrum foregrounds within the bounds ofthe maximum geometric delays physically realizable on that baseline. Byfocusing on delay-modes that correspond to image-domain regions beyond thehorizon, we show that it is possible to avoid the bulk of smooth-spectrumforegrounds. We map the point-spread function of delay-modes to k-space,showing that delay-modes that are uncorrupted by foregrounds also representsamples of the three-dimensional power spectrum, and can be used to constraincosmic reionization. Because it uses only spectral smoothness to differentiateforegrounds from the targeted 21cm signature, this per-baseline analysisapproach relies on spectrally- and spatially-smooth instrumental responses forforeground removal. For sufficient levels of instrumental smoothness relativeto interfering foregrounds, this technique substantially reduces the level ofcalibration previously thought necessary to detect 21cm reionization. As aresult, this approach places fewer constraints on antenna configuration withinan array, facilitating the adoption of configurations optimized forpower-spectrum sensitivity. Under these assumptions, we demonstrate thepotential for PAPER to detect 21cm reionization at an amplitude of 10 mK^2 neark~0.2h Mpc^-1 with 132 dipoles in 7 months of observing.

A Per-Baseline, Delay-Spectrum Technique for Accessing the 21cm Cosmic Reionization Signature