This paper presents the architecture and performance analysis of 5G ultra-dense cellular networks. The authors propose a distributed network architecture with single and multiple gateways for 5G ultra-dense cellular networks. The architecture is designed to support the high density of small cells required for 5G networks. The paper investigates the impact of small cell density on backhaul network capacity and backhaul energy efficiency. Simulation results show that there is a density threshold for small cells in ultra-dense cellular networks. When the density of small cells exceeds this threshold, the backhaul network capacity and backhaul energy efficiency decrease. The paper also discusses the challenges of 5G ultra-dense cellular networks, including multi-hop relay optimization, cooperative transmission, and the impact of massive MIMO and millimeter wave communication technologies on energy efficiency. The authors conclude that 5G ultra-dense cellular networks are density-limited communication systems, and the optimal deployment of small cells needs to be determined. The paper also highlights the importance of energy efficiency in 5G networks and the need for new energy efficiency models. The authors acknowledge the support of various funding agencies for this research.This paper presents the architecture and performance analysis of 5G ultra-dense cellular networks. The authors propose a distributed network architecture with single and multiple gateways for 5G ultra-dense cellular networks. The architecture is designed to support the high density of small cells required for 5G networks. The paper investigates the impact of small cell density on backhaul network capacity and backhaul energy efficiency. Simulation results show that there is a density threshold for small cells in ultra-dense cellular networks. When the density of small cells exceeds this threshold, the backhaul network capacity and backhaul energy efficiency decrease. The paper also discusses the challenges of 5G ultra-dense cellular networks, including multi-hop relay optimization, cooperative transmission, and the impact of massive MIMO and millimeter wave communication technologies on energy efficiency. The authors conclude that 5G ultra-dense cellular networks are density-limited communication systems, and the optimal deployment of small cells needs to be determined. The paper also highlights the importance of energy efficiency in 5G networks and the need for new energy efficiency models. The authors acknowledge the support of various funding agencies for this research.