  A great deal of efforts have been made on the performance evaluation of
distributed antenna systems (DASs). Most of them assume a regular base-station
(BS) antenna layout where the number of BS antennas is usually small. With the
growing interest in cellular systems with large antenna arrays at BSs, it
becomes increasingly important for us to study how the BS antenna layout
affects the rate performance when a massive number of BS antennas are employed.
  This paper presents a comparative study of the asymptotic rate performance of
downlink multi-user systems with multiple BS antennas either co-located or
uniformly distributed within a circular cell. Two representative linear
precoding schemes, maximum ratio transmission (MRT) and zero-forcing
beamforming (ZFBF), are considered, with which the effect of BS antenna layout
on the rate performance is characterized. The analysis shows that as the number
of BS antennas $L$ and the number of users $K$ grow infinitely while
$L/K{\rightarrow}\upsilon$, the asymptotic average user rates with the
co-located antenna (CA) layout for both MRT and ZFBF are logarithmic functions
of the ratio $\upsilon$. With the distributed antenna (DA) layout, in contrast,
the scaling behavior of the average user rate closely depends on the precoding
schemes. With ZFBF, for instance, the average user rate grows unboundedly as
$L, K{\rightarrow} \infty$ and $L/K{\rightarrow}\upsilon{>}1$, which indicates
that substantial rate gains over the CA layout can be achieved when the number
of BS antennas $L$ is large. The gain, nevertheless, becomes marginal when MRT
is adopted.
