  We present a detailed analysis of post-correlation beamforming (i.e.
beamforming which involves only phased sums of the correlation of the voltages
of different antennas in an array), and compare it with the traditionally used
incoherent and phased beamforming techniques. Using data from the GMRT we show
that post-correlation beamformation results in a many-folds increase in the
signal-to-noise for periodic signals from pulsars and several order of
magnitude reduction in the number of false triggers from single pulse events
like fast radio bursts (FRBs). This difference arises primarily because the
post-correlation beam contains less red-noise, as well as less radio frequency
interference. The post-correlation beam can also be more easily calibrated than
the incoherent or phased array beams. We also discuss two different modes of
post-correlation beamformation, viz. (1) by subtracting the incoherent beam
from the coherent beam and (2) by phased addition of the visibilities. The
computational costs for both these beamformation techniques as well as their
suitability for studies of pulsars and FRBs are discussed. Techniques discussed
here would be of interest for all upcoming surveys with interferometric arrays.
Finally, we describe a time-domain survey with the GMRT using the
post-correlation beamformation as a case study. We find that post-correlation
beamforming will improve the current GMRT time-domain survey sensitivity by ~ 2
times for pulsars with periods of few 100s of millisecond and by many-folds for
even slower pulsars, making it one of the most sensitive surveys for pulsars
and FRBs at low and mid radio frequencies.
