A new interleaved narrow beam tri-sector cell (INBTC) architecture is proposed to to improve the spectrum efficiency and channel quality for personal communications services (PCS) with low infrastructure costs. Combining the INBTC architecture and the newly developed interleaved channel assignment (ICA) scheme, we introduce the concept of interleaved cluster, within which a channel set can be assigned more than once. The INBTC architecture with interleaved clusters can enhance the performance of cellular/PCS systems from several perspectives. We demonstrate that, to design low reuse factors to achieve high spectrum efficiency, specifically in the range of reuse factors N=2∼7, the INBTC system improves 3∼5 dB at the 90th and 95th percentiles of signal-to-interference ratio (SIR) compared to conventional cellular systems. For a typical cellular environment, with a shadow fading standard deviation σ=8 dB and an SIR requirement of 17 dB at the 90th percentile, the INBTC system can increase the system capacity by 75% over current systems. Because the INBTC architecture can utilize the base station equipment of current cellular/PCS systems, implementing it should be simple and carry no extra infrastructure costs. Hence, the proposed INBTC system (the cell layout plus the channel assignment scheme) has significant potential value in future cellular/PCS systems, as well as in broadband wireless applications.