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5G Ultra-Dense Cellular Networks

Xiaohu Ge, Song Tu, Guoqiang Mao, Cheng-Xiang Wang, Tao Han

arXiv:1512.03143v1cs.NI

TL;DR

The paper asks how densely small cells can be deployed in 5G ultra-dense cellular networks while respecting backhaul constraints. It proposes distributed architectures and studies backhaul capacity and energy efficiency, finding density thresholds beyond which both decline.

  • Problem

    The paper addresses how much densification can be deployed in 5G ultra-dense cellular networks under backhaul network capacity and energy-efficiency constraints.

  • Method

    The paper presents single- and multiple-gateway distributed architectures and investigates small-cell density effects on backhaul capacity and energy efficiency.

  • Results

    A small-cell density threshold exists; beyond it, further density reduces backhaul network capacity and backhaul energy efficiency.

  • Takeaways & Limitations

    The 5G ultra-dense cellular network is a density-limited communication system, and the results provide guidelines for optimum deployment.

Abstract

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Traditional ultra-dense wireless networks are recommended as a complement for cellular networks and are deployed in partial areas, such as hotspot and indoor scenarios. Based on the massive multiple-input multi-output (MIMO) antennas and the millimeter wavecommunication technologies, the 5G ultra-dense cellular network is proposed to deploy in overall cellular scenarios. Moreover, a distribution network architecture is presented for 5G ultra-dense cellular networks. Furthermore, the backhaul network capacity and the backhaul energy efficiency of ultra-dense cellular networks are investigated to answer an important question, i.e., how much densification can be deployed for 5G ultra-dense cellular networks. Simulation results reveal that there exist densification limits for 5G ultra-dense cellualr networks with backhaul network capacity and backhaul energy efficiency constraints.

I. INTRODUCTION

5G ultra-dense cellular networks extend densification across cellular scenarios using massive MIMO and millimeter-wave technologies. The paper investigates distributed architectures and backhaul limits to determine how densely small cells can be deployed.

  • Massive MIMO and millimeter-wave communication motivate 5G small-cell deployment through high antenna capacity and broad bandwidth, but shorter transmission ranges.Millimeter-wave transmission distance is restricted to 100 meters because of atmospheric propagation degradation.
  • 5G cellular networks are expected to become ultra-dense, with small-cell densities anticipated at 40-50 BS/km2.
  • System-level investigation of ultra-dense cellular networks with millimeter-wave backhaul and distributed architecture performance limits remains lacking.
  • The paper proposes distributed architectures with single and multiple gateways and investigates how small-cell density affects backhaul capacity and energy efficiency.
  • A density threshold exists: beyond it, further small-cell densification reduces both backhaul network capacity and backhaul energy efficiency.

II. ARCHITECTURE OF 5G ULTRA-DENSE CELLULAR NETWORKS

The paper identifies architecture design as a first challenge for 5G ultra-dense cellular networks and proposes distribution architectures with single and multiple gateways for evaluation.

  • Massive MIMO and millimeter-wave technologies are expected to produce 5G ultra-dense cellular networks with many deployed small cells.
  • The paper proposes distribution architectures with single and multiple gateways for 5G ultra-dense cellular networks.

A. Conventional Cellular Network Architecture

Conventional cellular networks use tree-based architectures in which base stations forward traffic through gateways or core-network links. Microcells complement macrocells by serving indoor and hotspot regions.

  • The conventional cellular architecture is a tree network in which each macrocell BS independently transmits user and management data.
  • Microcell backhaul traffic reaches the core network through a gateway, broadband Internet, or fiber links.
  • Macrocell and microcell managers in the core network control handover between macrocells and microcells.
  • Microcells complement macrocells by providing high-speed wireless transmission in indoor and hotspot scenarios.

B. Distribution Architecture of Ultra-Dense Cellular Networks

The proposed 5G ultra-dense network jointly uses macrocells and small cells with distributed backhaul architectures. Multi-hop links address millimeter-wave range and deployment constraints while macrocells manage handover.

  • Forwarding every small-cell BS directly through broadband Internet or fiber is difficult because of cost, geography, and urban deployment constraints.
  • Millimeter-wave small-cell BSs cannot usually reach the gateway directly because wireless transmission distance is restricted.
  • The paper proposes single- and multiple-gateway distribution architectures for evaluating 5G ultra-dense cellular networks.
  • Distributed architecture relays wireless backhaul traffic to the gateway through multi-hop links.
  • The macrocell BS transmits management data for controlling user handover in small cells.
  • 5G ultra-dense cellular networks jointly comprise small cells and macrocells rather than treating small cells only as a complement.

1) Ultra-Dense Cellular Networks with Single Gateway:

The single-gateway architecture places a gateway at the macrocell BS and uses multi-hop millimeter-wave links to aggregate small-cell backhaul traffic before forwarding it to the core network.

  • 1) Ultra-Dense Cellular Networks with Single Gateway:: A gateway is deployed at the macrocell BS, which receives wireless backhaul traffic from small cells using massive MIMO millimeter-wave antennas.
  • 1) Ultra-Dense Cellular Networks with Single Gateway:: Small-cell BSs relay backhaul traffic to adjacent small-cell BSs through millimeter-wave links.
  • 1) Ultra-Dense Cellular Networks with Single Gateway:: All small-cell backhaul traffic is forwarded to the macrocell BS through multi-hop millimeter-wave links.
  • 1) Ultra-Dense Cellular Networks with Single Gateway:: The macrocell BS forwards aggregated backhaul traffic to the core network through fiber to the cell links.

2) Ultra-Dense Cellular Networks with Multiply Gateways:

The multiply-gateway architecture distributes backhaul traffic across gateways deployed at selected small-cell BSs, supporting flexible forwarding in dense 5G deployments.

  • 2) Ultra-Dense Cellular Networks with Multiply Gateways:: Gateways are deployed at multiple small-cell BSs according to backhaul traffic requirements and geographic deployment scenarios.
  • 2) Ultra-Dense Cellular Networks with Multiply Gateways:: Small-cell backhaul traffic is relayed through millimeter-wave links and distributed among multiple gateways in the macrocell.
  • 2) Ultra-Dense Cellular Networks with Multiply Gateways:: Traffic aggregated at each gateway is forwarded to the core network through fiber to the cell links.
  • 2) Ultra-Dense Cellular Networks with Multiply Gateways:: 5G ultra-dense cellular networks use massive MIMO and millimeter-wave technologies, which reduce small-cell coverage and require many small cells for seamless coverage.
  • 2) Ultra-Dense Cellular Networks with Multiply Gateways:: The distributed architecture supports high-bit-rate coverage across cellular regions while separating macrocell management data from small-cell user-data transmission.
  • 2) Ultra-Dense Cellular Networks with Multiply Gateways:: Single-gateway networks are cost efficient but may experience a gateway capacity bottleneck, whereas multiple gateways require higher small-cell deployment cost.

III. BACKHAUL NETWORK CAPACITY AND BACKHAUL ENERGY EFFICIENCY

Because infinite small-cell density is impractical, the paper investigates how ultra-dense deployment density affects backhaul capacity and energy efficiency.

  • III. BACKHAUL NETWORK CAPACITY AND BACKHAUL ENERGY EFFICIENCY: Although theory may allow small-cell density to approach infinity, infinite-density deployment is unrealistic in practical engineering applications.
  • III. BACKHAUL NETWORK CAPACITY AND BACKHAUL ENERGY EFFICIENCY: The comparison concerns conventional cellular networks and 5G ultra-dense cellular networks.
  • III. BACKHAUL NETWORK CAPACITY AND BACKHAUL ENERGY EFFICIENCY: The study investigates the impact of ultra-dense network deployment density on backhaul network capacity and backhaul energy efficiency.

A. Backhaul Network Capacity of Ultra-Dense Cellular Networks

The paper models and simulates multi-hop wireless backhaul capacity in a multi-gateway 5G ultra-dense network. Capacity rises initially with densification or simultaneous transmissions, then reaches a maximum or saturation limit and can decline with further densification.

  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: The key design question is how much densification can be deployed in ultra-dense cellular networks for 5G.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: The multi-hop backhaul scheme selects the closest gateway, relays through eligible small-cell BSs, and transmits directly when a BS is within radius r of its gateway.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: Backhaul network capacity is estimated as Y(n) × W, where n is the number of small-cell BSs, Y(n) the average simultaneous transmissions, and W the small-cell transmission rate.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: With fixed small-cell radius, capacity first increases with small-cell BS count, then decreases after a maximum threshold, and finally approaches stationary saturation.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: 29, 25 and 19 maximum average simultaneous transmissions occur for small-cell radii of 100 m, 150 m and 200 m, respectively.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: Capacity approaches saturation when simultaneous transmissions exceed 27, 23 and 15 for small-cell radii of 100 m, 150 m and 200 m, respectively.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: For fixed BS count or simultaneous transmissions, backhaul capacity decreases as small-cell radius increases.
  • A. Backhaul Network Capacity of Ultra-Dense Cellular Networks: The results indicate that backhaul capacity becomes a bottleneck constraining small-cell densification beyond a given density threshold.

B. Backhaul Energy Efficiency of Ultra-Dense Cellular Networks

The paper models backhaul energy efficiency using small-cell backhaul throughput, operating power, lifetime, and energy components. Efficiency varies non-monotonically with network densification, cell radius, and average small-cell throughput, revealing saturation behavior and operating thresholds.

  • Energy-efficiency model: Backhaul energy efficiency is constrained by network capacity and is modeled from small-cell backhaul transmission and operating power over a five-year lifetime.Operating power is expressed as POP = a×PTX+b, with a = 7.84 and b = 71.5 Watt.
  • Energy-efficiency model: The model derives average small-cell backhaul transmission power from throughput, then computes operating power and backhaul energy efficiency.PTX scales with average throughput relative to a 1 Gbps reference, while embodied energy accounts for 20% of backhaul BS energy consumption.
  • Densification effects: With fixed cell radius, efficiency first increases with the number of small-cell BSs, then decreases after a maximum threshold, and finally reaches stationary saturation.This behavior is analyzed in Figure 4(a).
  • Densification effects: For fixed small-cell count, efficiency increases with radius below 10 BSs but decreases with radius when the count is at least 10.The radius therefore interacts with network density in determining backhaul energy efficiency.

IV. FUTURE CHALLENGES

The paper identifies architectural, mobility, resource-allocation, and energy-modeling challenges for 5G ultra-dense cellular networks. It highlights multi-hop relaying, cooperative transmission, massive-MIMO resource allocation, and computation power as future research directions.

  • Future challenges: Multi-hop relay optimization must account for relaying both backhaul and fronthaul traffic and carefully select relaying small-cell BSs.The paper proposes developing new multi-hop relay schemes and distribution routing algorithms.
  • Future challenges: Small-cell coverage and millimeter-wave directivity complicate high-speed-user mobility, motivating cooperative transmission among adjacent small cells.The paper identifies dynamic grouping of small cells along a high-speed user track as an open issue.
  • Future challenges: Massive-MIMO beamforming increases baseband signal-processing scale and wireless-transceiver computation power.The paper states that computation power may become comparable to or exceed transmission power and therefore cannot be ignored in BS energy consumption.
  • Future challenges: A new energy-efficiency model is needed for ultra-dense networks using massive MIMO and millimeter-wave technologies.The paper also calls for rethinking computation power consumed by BS baseband processing systems.
  • Future challenges: Massive MIMO antennas create a resource-allocation problem between BS relaying and self-transmission.The paper describes this allocation as a critical problem in 5G ultra-dense cellular networks.

V. CONCLUSIONS

The paper presents distributed architectures for 5G ultra-dense cellular networks and investigates how small-cell density affects backhaul capacity and energy efficiency. The results identify density thresholds beyond which further densification reduces these backhaul measures, motivating deployment guidelines.

  • Massive MIMO and millimeter-wave communication enable 5G ultra-dense cellular networks across cellular scenarios.
  • The paper presents distributed network architectures with single and multiple gateways for 5G ultra-dense cellular networks.
  • The study investigates how small-cell BS density affects backhaul network capacity and backhaul energy efficiency.
  • A density threshold exists: beyond it, further increases in ultra-dense network density reduce backhaul network capacity and energy efficiency.
  • The paper concludes that 5G ultra-dense cellular networks are density-limited communication systems and provides guidelines for optimum deployment.
  • Analytically determining the optimum density of small-cell BSs remains an open issue.
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