Capacity of present narrowband cellular systems: interference-limited or blocking-limited

Abstract

There is a widespread notion within the North American cellular industry that present cellular systems are intrinsically interference-limited. This idea in part comes from a number of reported simulations which were based on the assumption that mobiles always connect to the nearest base station. We present analytical and simulation results for the case where mobiles are connected to the best base station (the one with the strongest signal). At the 10 percent outage level, it is shown that the nearest server scenario tends to underestimate the carrier-to-interference (C/I) ratio by as much as 5 dB (with respect to the best server case). Some system imperfections which are not traditionally assessed by such simulations are further discussed. In particular, nonideal positioning of the base stations was found to have negligible impact on C/I if the traditional cellular rule-of-thumb criteria are obeyed. Nonuniform spatial distribution of the mobiles was analyzed based on real data from the Toronto area. It was found to have negligible impact on the average C/I performance. Blocking problems in such a case, however, could easily reduce the theoretical capacity with uniform loading by as much as 30 percent. Finally, C/I deteriorations due to nonideal handoffs are examined. This analysis is very sensitive to the handoff mechanism implementation details. Although the results for hand-offs should be regarded with some caution, in view of all the results presented in this article we feel that cellular systems are in general limited by blocking rather than by interference.