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Evolution Towards 5G Multi-tier Cellular Wireless Networks: An Interference Management Perspective

E. Hossain, M. Rasti, H. Tabassum, A. Abdelnasser

arXiv:1401.5530v2cs.NI

TL;DR

5G multi-tier networks create an interference-management gap because heterogeneous users and tiers share spectrum while having different access priorities. This article surveys and qualitatively compares distributed cell association and power-control schemes, then identifies limitations and modification guidelines. It concludes that existing schemes do not adequately protect high-priority users and should be adapted to enforce interference thresholds while pursuing lower-priority users’ objectives.

  • Problem

    Existing interference-management schemes are not sufficient for prioritized 5G multi-tier networks, where different tiers share spectrum and users have different channel-access priorities.

  • Method

    The article outlines 5G requirements, surveys and qualitatively compares distributed cell association and power-control schemes, and proposes modification guidelines.

  • Results

    Existing distributed power-control schemes do not guarantee that interference from low-priority users stays within tolerable limits for high-priority users, potentially causing high-priority SIR outage.

  • Takeaways & Limitations

    Prioritized schemes should limit low-priority transmit power below predefined interference thresholds while allowing them to track their own objectives.

Abstract

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The evolving fifth generation (5G) cellular wireless networks are envisioned to overcome the fundamental challenges of existing cellular networks, e.g., higher data rates, excellent end-to-end performance and user-coverage in hot-spots and crowded areas with lower latency, energy consumption and cost per information transfer. To address these challenges, 5G systems need to adopt a multi-tier architecture consisting of macrocells, different types of licensed small cells, relays, and device-to-device (D2D) networks to serve users with different quality-of-service (QoS) requirements in a spectrum and energy-efficient manner. Starting with the visions and requirements of 5G multi-tier networks, this article outlines the challenges of interference management (e.g., power control, cell association) in these networks with shared spectrum access (i.e., when the different network tiers shares the same licensed spectrum). It is argued that the existing interference management schemes will not be able to address the interference management problem in prioritized 5G multi-tier networks where users in different tiers have different priorities for channel access. In this context, a survey and qualitative comparison of the potential existing cell association and power control schemes is provided to demonstrate their limitations for interference management in 5G networks. Open challenges are highlighted and guidelines are provided to modify the existing schemes in order to overcome these limitations and make them suitable for the emerging 5G systems.

I. INTRODUCTION

5G is envisioned as a heterogeneous, multi-tier system designed to meet sharply increasing traffic and performance demands. Its requirements include higher capacity and data rates, lower latency and energy consumption, broader device support, and shared access across diverse technologies.

  • I. INTRODUCTION: 5G targets 1000 times higher mobile data volume per unit area, 10-100 times more connected devices and user data rate, 10 times longer battery life, and 5 times lower latency.A data rate of about 10 Gbps is also identified as a key 5G attribute.
  • I. INTRODUCTION: 5G combines network tiers of different sizes and powers, multiple RATs, diverse backhaul connections, and heterogeneous wireless devices.The architecture includes macrocells, small cells, relays, and D2D links.
  • I. INTRODUCTION: In dense urban areas, 5G is envisioned to provide 300 Mbps downlink and 60 Mbps uplink rates in 95% of locations and time, with 2 to 5 millisecond end-to-end latency.
  • I. INTRODUCTION: Prioritized spectrum access distinguishes traffic-based priorities from tier-based priorities, with protection requirements potentially reversing between downlink and uplink.D2D access is allowed when interference to cellular users remains below a given threshold.
  • I. INTRODUCTION: 5G requirements include improved energy efficiency for battery-constrained devices, additional millimeter-wave spectrum, and continued evolution of existing RATs alongside new technologies.Energy harvesting and SWIPT are discussed, although practical harvesting circuits are not yet available.

III. INTERFERENCE MANAGEMENT CHALLENGES IN 5G MULTI-TIER NETWORKS

Interference management becomes difficult in 5G multi-tier networks because heterogeneous, densely deployed tiers have unequal powers, access restrictions, QoS requirements, and priorities. The article therefore calls for extensions to cell association and power control that support multi-connectivity, coordination, and prioritized protection.

  • III. INTERFERENCE MANAGEMENT CHALLENGES IN 5G MULTI-TIER NETWORKS: Interference dynamics are affected by heterogeneous dense deployments, unequal downlink transmit powers, access restrictions, and different access priorities.
  • III. INTERFERENCE MANAGEMENT CHALLENGES IN 5G MULTI-TIER NETWORKS: The network architecture contains macrocells, picocells, femtocells, relays, and D2D links connected by wireless links and backhaul connections.
  • III. INTERFERENCE MANAGEMENT CHALLENGES IN 5G MULTI-TIER NETWORKS: Cell association and power control should jointly address interference mitigation, throughput, power consumption, link quality, and congestion rather than relying only on SIR.
  • III. INTERFERENCE MANAGEMENT CHALLENGES IN 5G MULTI-TIER NETWORKS: Existing single-BS association schemes must be extended to support simultaneous connectivity to multiple BSs, particularly for cell-edge users.The stated goals are higher throughput and lower outage through more effective resource use.
  • III. INTERFERENCE MANAGEMENT CHALLENGES IN 5G MULTI-TIER NETWORKS: Cooperation among tiers can mitigate interference, but it requires reliable, fast, low-latency backhaul integration of low-power nodes.The article identifies this backhaul requirement as a major technical issue for upcoming multi-tier 5G networks.

A. Distributed Cell Association Schemes

Distributed cell association schemes use received signal, biasing, or resource-related information to select serving cells, but their effectiveness depends on interference, scheduling, traffic load, and parameter choices. Standard schemes therefore require optimization for multi-tier operation.

  • A. Distributed Cell Association Schemes: RSRP-based association selects the BS with the largest average received signal strength, while RSRQ-based selection resembles SIR-based association.
  • A. Distributed Cell Association Schemes: CRE increases low-power BS coverage by adding a positive bias to received signal strength, encouraging user offloading for downlink load balancing.
  • A. Distributed Cell Association Schemes: ABS-based association uses blank subframes at an unbiased BS to schedule off-loaded users and reduce inter-tier interference.Greater bias causes more offloading and therefore requires more blank subframes to protect off-loaded users.
  • A. Distributed Cell Association Schemes: Channel-aware, interference-aware, load-aware, resource-aware, and priority-aware schemes differ in the information they require and the protections they provide.Priority-aware schemes use tier-priority information to protect high-priority users.
  • A. Distributed Cell Association Schemes: Standard RSRP, RSRQ, and CRE schemes cannot guarantee optimum multi-tier performance unless bias, transmit power, resource partitioning, and related parameters are optimized.

B. Distributed Power Control Schemes

Distributed power-control schemes pursue different objectives, including target-SIR satisfaction, outage reduction, throughput maximization, and dynamic resource use. However, existing schemes do not protect high-priority users in prioritized 5G multi-tier networks.

  • Scheme categories: Power-control schemes are classified by their objectives and constraints, including target-SIR tracking, gradual removal, opportunistic allocation, and dynamic-SIR tracking.The article compares these schemes with corresponding distributed solutions for prioritized 5G multi-tier networks.
  • Target-SIR tracking: Target-SIR tracking achieves fixed target-SIRs with minimal aggregate transmit power when the targets are feasible.Its operation assumes that the users’ target-SIR requirements can be jointly satisfied.
  • Infeasibility: When target-SIR requirements are infeasible, unsupported users transmit at maximum power, increasing power consumption, interference, and the number of unsupported users.Gradual-removal variants address infeasibility by reducing unsupported users’ power or removing them progressively.
  • Opportunistic allocation: Opportunistic power control favors users with good channels, improving system performance at the cost of reduced fairness.Small path-gain differences can produce large throughput differences between users.
  • Dynamic-SIR tracking: Dynamic-SIR tracking can improve system throughput over fixed-target tracking when additional resources are available, but it consumes more power.Fixed target-SIR assignment can prevent users from exploiting resources that would support higher SIRs and throughputs.
  • Prioritized-network limitation: Existing distributed power-control schemes cannot guarantee that low-priority users’ interference remains tolerable for high-priority users.The resulting high-priority-user SIR outage motivates power limits that preserve a specified interference threshold while low-priority users pursue their objectives.

C. Joint Cell Association and Power Control Schemes

Joint cell association and power control has been studied in distributed conventional-network frameworks with convergence guarantees. These frameworks may require modification for 5G multi-tier networks because interference depends on access protocols, QoS requirements, and tier priorities.

  • Existing joint frameworks: A distributed uplink framework jointly selects cells using effective interference and minimizes aggregate transmit power while meeting users’ desired SIR targets.The framework is described for single-tier networks and includes guaranteed convergence.
  • Compatibility limits: Conventional distributed CAPC frameworks may not be directly compatible with 5G multi-tier networks.Their suitability is limited by the different interference dynamics of multi-tier deployments.
  • Required extensions: 5G CAPC optimization should incorporate multiple cell-selection methods and power-control objectives with interference constraints tailored to macrocell, picocell, and D2D receivers.The required constraints depend on channel-access protocols, scheduling, QoS requirements, and tier priorities.

V. DESIGN GUIDELINES FOR DISTRIBUTED CAPC SCHEMES IN 5G MULTI-TIER NETWORKS

Distributed CAPC schemes for 5G networks should support multi-BS connectivity, cell-load balancing, and interference protection for high-priority users.

  • Multi-BS association: Users may need simultaneous connections to multiple BSs, with associations differing between uplink and downlink.This requirement is part of the proposed direction for distributed 5G interference management.
  • Load and interference management: Distributed CAPC should achieve load balancing across cells while guaranteeing interference protection for high-priority users.The guideline combines association flexibility with tier-aware interference management.
  • Design direction: The article presents these requirements as guidelines for modifying existing CAPC schemes in 5G multi-tier networks.The proposed direction targets the combined challenges of connectivity, cell load, and priority protection.

A. Prioritized Power Control

Prioritized power control modifies existing schemes so low-priority users pursue their objectives while limiting interference to high-priority receivers. In the evaluated network, this protection is achieved by sacrificing some low-priority performance.

  • Prioritized power-control mechanism: Prioritized power control limits low-priority users’ transmit power so interference at high-priority users remains below a predefined threshold.High-priority users can send commands to nearby low-priority users when interference exceeds the threshold.
  • Objective protection: The proposed modification lets low-priority users track their own objectives while maintaining interference protection for high-priority users.The approach can be applied to existing distributed power-control algorithms after adding the interference constraint.
  • Evaluation setting: The evaluation uses 3 × 3 macro cells with n = 3, 4, 5, 6 small cells per macrocell and averages results over 100 snapshots.Macrocell areas are 1000 m × 1000 m, while small-cell areas are 200 m × 200 m.
  • Evaluation findings: Prioritized TPC and TPC-GR guarantee high-priority-user protection at the cost of increased low-priority outage, while prioritized OPC reduces low-priority throughput.The non-prioritized TPC and TPC-GR schemes do not guarantee high-priority protection in the comparison.

B. Resource-Aware Cell Association Schemes

Resource-aware association adapts cell selection to traffic and channel-access conditions, while hybrid association combines resource awareness with distance-based channel quality. In simulations, the hybrid scheme improves per-channel spectral efficiency for typical users, especially in sparse deployments.

  • Resource-aware association: Resource-aware association selects the base station with maximum channel access probability, adapting to each cell’s scheduling criterion.The access probability varies with resource allocation; under round-robin scheduling it is the reciprocal of the number of users.
  • Resource-aware association: Under greedy scheduling, channel access probability depends on both channel gain and the number of users in the cell.As cell occupancy grows, the probability decreases, discouraging further associations and implicitly balancing traffic load.
  • Hybrid association: Distance-aware association tends to improve sum-rate performance but does not account for traffic load.The hybrid scheme combines distance-based channel gain with channel access probability.
  • Evaluation setup: The comparison evaluates distance-aware, resource-aware, and hybrid association in a circular macrocell overlaid by randomly deployed small cells.The setup uses path-loss exponent 4, macrocell transmit power 10 W, and small-cell transmit power 1 W, with non-uniform traffic loads.
  • Results: Resource-aware association can significantly degrade spectral efficiency because of strong nearby interferers, whereas hybrid association provides significant per-channel gains, especially in sparse deployments.For round-robin scheduling, resource-aware association is equivalent to load-aware association.

C. Resource-Aware Cell Association and Prioritized Power Control

Cell association can be combined with prioritized power control, but the pairing must match the desired objective. For uplink throughput maximization, combining minimum effective-interference association with opportunistic power control can drive users toward high transmit powers and requires channel-condition-based alternatives.

  • Joint CAPC design: Cell-association methods can be combined with prioritized power-control schemes according to the desired objective.Selecting a suitable combination is an important design issue in prioritized multi-tier networks.
  • Joint CAPC design: Minimum effective-interference association with opportunistic power control cannot address uplink throughput maximization.Users associate with the base station having minimum effective interference, which ultimately results in high transmit power for all users.
  • Joint CAPC design: With opportunistic power control, increasing transmit power helps users with good channels but degrades throughput when users with poor channels increase power.The association rule therefore needs to account for channel conditions rather than only received interference.
  • Joint CAPC design: RSRP- or RSRQ-based association may be useful with opportunistic power control because it bases associations on channel conditions.These approaches are suggested as alternatives to minimum effective-interference association for the throughput objective.

VI. CONCLUSION

The paper identifies interference-management challenges created by prioritized access, simultaneous multi-base-station connectivity, and different uplink and downlink associations in 5G multi-tier networks. It recommends modifying existing schemes and developing joint CAPC methods for multiple objectives.

  • Conclusion: 5G multi-tier networks introduce different access priorities for users and tiers, simultaneous connectivity to multiple base stations, and distinct uplink and downlink associations.These features create new interference-management challenges while increasing the degrees of freedom for power control and cell association.
  • Conclusion: The paper highlights open challenges and provides guidelines for modifying existing cell-association and power-control schemes for 5G multi-tier networks.The proposed direction is to develop efficient joint CAPC methods.
  • Conclusion: Future joint CAPC methods should address objectives such as throughput maximization, traffic-load balancing, and minimum SIR for high-priority users.The paper identifies resource-aware association combined with conventional association methods and prioritized power control as a promising direction.
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