Coverage Optimization Trade-Offs in Heterogeneous W-CDMA Networks with Co-Channel Small Cells

This study examines the trade-offs related to the problem of coverage auto-configuration of open access small cells (SC) that (i) are on-demand deployed in the macro-cells (MC) of a W-CDMA network, and (ii) operate in the same frequency channel as the hosting/neighboring MCs. The goal is to identify stable and practically detectable equilibrium states whose achievement can represent an optimality criterion for implementing automatic coverage adaptation of the SCs. It is shown that, on the downlink (DL), the offloading of MC hotspot users by a nearby SC improves the DL signal-to-interference-and-noise ratio (SINR) and the DL effective throughput of the MC users at the expense of the SC users. Similarly on the uplink (UL), the maximum achievable UL SINR of the SC (MC) users decreases (increases) with a growing SC coverage, but a distinct equilibrium state of the MC-vs-SC trade-off defined on the basis of the spectral radius of a network information matrix can be observed. However, the DL and UL equilibria do not generally coincide unless the MC and SC performance requirements are identical. This fact complicates autonomous optimum partitioning of the SC power budget between data transmissions and coverage-defining pilot signals. Numerical simulations are based on a 3GPP-compliant system model.