Capacity and coverage in two-tier cellular CDMA networks.

摘要

Wireless operators often encounter small regions of dense user traffic within a larger coverage area, i.e., a macrocell. A practical solution for alleviating the resulting capacity shortage is the installation of hotspot microcells. This research studies the capacity and coverage of such a two-tier architecture in a single-carrier CDMA system. Analytic methods are presented to compute both the uplink and downlink performance. These methods account for the effects of propagation parameters, system design parameters, power control strategies, user spatial distribution, base station selection criteria, soft-handoff, voice activity detection, limited terminal power, and channel dispersion.; First, a system operating in ideal conditions is examined. These ideal conditions stipulate that users and base stations transmit over an infinitely dispersive channel so that their received signal powers remain constant. Additionally, they require that user terminals (1) do not have maximum transmit power constraints; and (2) employ ideal multiuser detectors to cancel any downlink in-cell interference. A simple two-cell system composed of one macrocell and one microcell is studied under such idealized conditions. Exact and approximate analyses are presented to compute the user capacity, the number of simultaneous voice users supported with a specified reliability, on the uplink; the results are then corroborated using simulation methods. The downlink user capacity under the assumed idealized conditions is shown to exceed that of the uplink. Also studied for the uplink user capacity of the idealized two-cell system are: soft-handoff, voice activity detection, sensitivity to propagation parameters, and a specific architecture in which the microcell operates as a data access point (DAP). The desensitivity parameter, δ, is invoked as a way to optimize the data capacity for this microcell application. The analysis of the simple two-cell system is then extended to determine uplink user capacity in a system composed multiple macrocells and multiple microcells. A linear growth in capacity with the number of microcells (L) is shown to apply at small L, and to be extendable as L grows via changes in δ.; The second portion of the study determines the performance of the simple two-cell system under non-ideal conditions. In the uplink direction, the effects of transmit power constraints and channel fading are thoroughly investigated. Then, the non-ideal downlink is studied, wherein power fading and in-cell interference degrade performance. We identify the conditions under which user capacity is limited by the downlink rather than the uplink, and vice versa.; In most of the cases examined, simple analytical techniques are devised and corroborated by simulations, and design rules are proposed to improve capacity and coverage. In addition, a large set of quantitative findings is established relating system and channel parameters to the performance of two-tier CDMA systems. Finally, several areas for further research are proposed.