Performance study of soft handover with CDMA heterogeneous cellular architectures

Abstract

This paper investigates the downlink user capacity of a heterogeneous cellular CDMA system with soft handover. We consider the scenario that a hotspot microcell is adjacent to a larger macrocell instead of a microcell embedded within a macrocell. Since traditional soft hand-off algorithms are developed for homogeneous cell structures, the two serving base stations allocate equal power in the downlink to the soft handover users [1]. We observe that this kind of equal power allocation will cause a serious "power exhausting problem" in the microcell when soft handover is occurred between two cells with different cell radius. To quantize the impact of this problem, we present an analytic approximation method for computing the downlink user capacity with soft handover in heterogeneous cellular structures. We further propose an improved quality balancing power allocation method with maximum power constraint to enhance the downlink soft handover performance when two involving handover base stations have different cell sizes. The simulation results show that the improved power allocation for soft handover can support a larger range of the ratios of the cell radius between microcell to macrocell, thus increasing the total system capacity in the sense that more microcells can be installed adjacent to a macrocells. Our numerical results demonstrate that in a heterogeneous CDMA network with the cell radius ratio between microcell and macrocell equal to 0.3, the proposed constrained unequal power allocation (UPA) technique with soft handover can enhance the total system capacity eight times higher than the conventional equal power allocation (EPA) method.