This paper develops a tractable, flexible, and accurate model for a downlink heterogeneous cellular network (HCN) consisting of \( K \) tiers of randomly located base stations (BSs), each tier differing in average transmit power, supported data rate, and BS density. The model assumes that a mobile user connects to the strongest candidate BS, and the resulting Signal-to-Interference-plus-Noise-Ratio (SINR) is greater than 1 when in coverage. The authors derive an expression for the probability of coverage (equivalently outage) over the entire network under both open and closed access, which simplifies to a closed-form expression in the high SINR regime and is accurate down to -4 dB even under weaker assumptions. They also derive the average data rate achieved by a randomly located mobile and the average load on each tier of BSs. The model is validated against an actual LTE network (for tier 1) with the other \( K-1 \) tiers modeled as independent Poisson Point Processes, showing accuracy within 1-2 dB. The paper highlights that in interference-limited open access networks, adding more tiers and/or BSs does not affect the probability of coverage or outage when all tiers have the same target SINR.This paper develops a tractable, flexible, and accurate model for a downlink heterogeneous cellular network (HCN) consisting of \( K \) tiers of randomly located base stations (BSs), each tier differing in average transmit power, supported data rate, and BS density. The model assumes that a mobile user connects to the strongest candidate BS, and the resulting Signal-to-Interference-plus-Noise-Ratio (SINR) is greater than 1 when in coverage. The authors derive an expression for the probability of coverage (equivalently outage) over the entire network under both open and closed access, which simplifies to a closed-form expression in the high SINR regime and is accurate down to -4 dB even under weaker assumptions. They also derive the average data rate achieved by a randomly located mobile and the average load on each tier of BSs. The model is validated against an actual LTE network (for tier 1) with the other \( K-1 \) tiers modeled as independent Poisson Point Processes, showing accuracy within 1-2 dB. The paper highlights that in interference-limited open access networks, adding more tiers and/or BSs does not affect the probability of coverage or outage when all tiers have the same target SINR.