Downlink System Throughput Statistics for Various MEA Configurations

The advent of applications that need higher throughputs motivates wireless service providers and cellular operators to embrace newer technologies that can meet these demands. Multiple-Input- Multiple-Output (MIMO) systems have shown promise in their ability to deliver high throughputs per bandwidth with reasonable constellation sizes. Adding antennas at the Base Station (BS) is practical due to reasons of size and cost amortization over users. However, adding antennas at the Mobile Station (MS), which does not have similar advantages, needs to be carefully evaluated. We therefore consider the more general class of techniques involving a multiple-element antenna (MEA) at one or both ends of the link (MIMO corresponding to the case of both). From a commercial standpoint, one needs to address the following questions: (i) What is the benefit of a second antenna at the BS or the MS relative to the Single-Input-Single- Output (SISO) case? (ii) What is the added value of a second antenna at both ends? (iii) If a second antenna is indeed used at both ends, which mode of operation - Spatial Multiplexing (SM) or Diversity (Div) - is the preferred one? Using (n, m) to denote a link with n BS transmit elements and m MS receive elements, we compare the downlink throughput performance of the SISO link with that of four MEA configurations: (1, 2), (2, 1), (2, 2) with Div and (2, 2) with SM. Our results indicate that, in the context of adaptive modulation with practical limits on constellation size, (1, 2) is the preferred configuration. We also show this finding to be robust to study assumptions.