  Large Scale Structures (LSS) in the universe can be traced using the neutral
atomic hydrogen HI through its 21cm emission. Such a 3D matter distribution map
can be used to test the Cosmological model and to constrain the Dark Energy
properties or its equation of state. A novel approach, called intensity mapping
can be used to map the HI distribution, using radio interferometers with large
instantaneous field of view and waveband. In this paper, we study the
sensitivity of different radio interferometer configurations, or multi-beam
instruments for the observation of large scale structures and BAO oscillations
in 21cm and we discuss the problem of foreground removal. For each
configuration, we determine instrument response by computing the (u,v) or
Fourier angular frequency plane coverage using visibilities. The (u,v) plane
response is the noise power spectrum, hence the instrument sensitivity for LSS
P(k) measurement. We describe also a simple foreground subtraction method to
separate LSS 21 cm signal from the foreground due to the galactic synchrotron
and radio sources emission. We have computed the noise power spectrum for
different instrument configuration as well as the extracted LSS power spectrum,
after separation of 21cm-LSS signal from the foregrounds. We have also obtained
the uncertainties on the Dark Energy parameters for an optimized 21 cm BAO
survey. We show that a radio instrument with few hundred simultaneous beams and
a collecting area of ~10000 m^2 will be able to detect BAO signal at redshift z
~ 1 and will be competitive with optical surveys.
