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

Asymptotic Rate Analysis of Downlink Multi-user Systems with Co-located and Distributed Antennas