  Despite substantial rate gains achieved by coordinated transmission from a
massive amount of geographically distributed antennas, the resulting
computational cost and channel measurement overhead could be unaffordable for a
large-scale distributed antenna system (DAS). A scalable signal processing
framework is therefore highly desirable, which, as recently demonstrated in
\cite{Dai_TWireless}, could be established based on the concept of virtual
cell.
  In a virtual-cell based DAS, each user chooses a few closest base-station
(BS) antennas to form its virtual cell, that is, its own serving BS antenna
set. In this paper, we focus on a downlink DAS with a large number of users and
BS antennas uniformly distributed in a certain area, and aim to study the
effect of the virtual cell size on the average user rate. Specifically, by
assuming that maximum ratio transmission (MRT) is adopted in each user's
virtual cell, the achievable ergodic rate of each user is derived as an
explicit function of the large-scale fading coefficients from all the users to
their virtual cells, and an upper-bound of the average user rate is
established, based on which a rule of thumb is developed for determining the
optimal virtual cell size to maximize the average user rate. The analysis is
further extended to consider multiple users grouped together and jointly served
by their virtual cells using zero-forcing beamforming (ZFBF). In contrast to
the no-grouping case where a small virtual cell size is preferred, it is shown
that by grouping users with overlapped virtual cells, the average user rate can
be significantly improved by increasing the virtual cell size, though at the
cost of a higher signal processing complexity.
