  Extending the formalism of Datta, Bharadwaj & Choudhury (2007) for detecting
ionized bubbles in redshifted 21 cm maps using a matched-filtering technique,
we use different simulations to analyze the impact of HI fluctuations outside
the bubble on the detectability of the bubble. In the first three kinds of
simulations there is a spherical bubble of comoving radius R_b, the one that we
are trying to detect, located at the center, and the neutral hydrogen (HI)
outside the bubble traces the underlying dark matter distribution. We consider
three different possible scenarios of reionization, i.e., (i) there is a single
bubble (SB) in the field of view (FoV) and the hydrogen neutral fraction is
constant outside this bubble (ii) patchy reionization with many small ionized
bubbles in the FoV (PR1) and (iii) many spherical ionized bubbles of the same
radius $R_b$ (PR2). The fourth kind of simulation uses more realistic maps
based on semi-numeric modelling (SM) of ionized regions. We find that for both
the SB and PR1 scenarios the fluctuating IGM restricts bubble detection to size
R_b<= 6 Mpc and R_b<= 12 Mpc for the GMRT and the MWA respectively, however
large be the integration time. These results are well explained by analytical
predictions. Large uncertainty due to the HI fluctuations restricts bubble
detection in the PR2 scenario for neutral fraction x_HI<0.6. The matched-filter
technique works well even when the targeted ionized bubble is non-spherical due
to surrounding bubbles and inhomogeneous recombination (SM). We find that
determining the size and positions of the bubbles is not limited by the HI
fluctuations in the SB and PR1 scenario but limited by the instrument's angular
resolution instead, and this can be done more precisely for larger bubble
(abridged).
