  Distribution of cold gas in the post-reionization era provides an important
link between distribution of galaxies and the process of star formation.
Redshifted 21 cm radiation from the Hyperfine transition of neutral Hydrogen
allows us to probe the neutral component of cold gas, most of which is to be
found in the interstellar medium of galaxies. Existing and upcoming radio
telescopes can probe the large scale distribution of neutral Hydrogen via HI
intensity mapping. In this paper we use an estimate of the HI power spectrum
derived using an ansatz to compute the expected signal from the large scale HI
distribution at z ~ 3. We find that the scale dependence of bias at small
scales makes a significant difference to the expected signal even at large
angular scales. We compare the predicted signal strength with the sensitivity
of radio telescopes that can observe such radiation and calculate the
observation time required for detecting neutral Hydrogen at these redshifts. We
find that OWFA (Ooty Wide Field Array) offers the best possibility to detect
neutral Hydrogen at z ~ 3 before the SKA (Square Kilometer Array) becomes
operational. We find that the OWFA should be able to make a 3 sigma or a more
significant detection in 2000 hours of observations at several angular scales.
Calculations done using the Fisher matrix approach indicate that a 5 sigma
detection of the binned HI power spectrum via measurement of the amplitude of
the HI power spectrum is possible in 1000 hours (Sarkar, Bharadwaj and Ali,
2017).
