The combination of macrocells and microcells in a two-tier cellular system is attractive because it provides a balance between maximizing the number of users per unit area and minimizing the network control associated with handoff. In this paper we study system capacity of a two-tier cellular architecture employing 3 TDMA/CDMA strategies. Both the uplink and downlink are evaluated via mathematical analysis. It shows that system performance degrades for larger macrocells or microcells and the capacities are poor for users near the macrocells' boundary. It also shows a huge discrepancy between the results obtained for the center cell versus those obtained for the boundary cell.

The CDMA system has the potential to provide more capacity than other systems and the hierarchical layer of cells will be popular in the future. However, the problem is whether the same RF channels used in CDMA overlayed/underlayed macrocell and microcell structures also obtain high capacity as in the homogeneous structure. This paper will investigate the uplink and downlink interference of microcell and macrocell systems under hierarchical conditions. The pilot signal and power control for downlinks are also considered. We use analytical and simulation methods to investigate the problem. However the results are discouraging. The capacity of microcells and macrocells is limited by the downlink and uplink respectively.