Undergraduate Thesis

Topic: Joint Transmission Coordinated Multipoint on Mobile Users in 5G Heterogeneous Networks

Published in IEEE 2nd Conference on Information Technology and Data Science (CITDS), Debrecen, Hungary, 2022

Overview

The fifth generation (5G) of mobile networks promises higher data rates, ultra-low latency, and massive device connectivity. However, these gains come with challenges. As networks become more heterogeneous (mixing macro, micro, and femto cells), users—particularly at the cell edges—face severe inter-cell interference (ICI) that degrades throughput and quality of service.

Another major issue arises from user mobility. At higher velocities (e.g., commuters on trains, vehicles on highways), Doppler shifts distort the transmitted signal, causing throughput degradation, reduced spectral efficiency, and unstable handovers.

This study investigates Joint Transmission Coordinated Multipoint (JT-CoMP) as a solution. JT-CoMP allows multiple base stations (eNodeBs) to coordinate transmissions so that a user receives constructive signals from different sources, effectively treating interference as useful signal. The research specifically evaluates intra-site and inter-site JT-CoMP performance in 5G Heterogeneous Networks (HetNets), while incorporating Closed Loop Spatial Multiplexing (CLSM) for adaptive signal processing.

Research Motivation and Goals

• ICI Challenge: HetNets, due to their density and varied power levels of cells, suffer from high ICI, limiting reliable coverage.

• Mobility Challenge: As user velocity increases, channel conditions fluctuate rapidly, making CSI (Channel State Information) estimation less accurate.

• Industry Need: Telecom operators require scalable, cost-effective solutions that boost capacity and coverage without drastically increasing transmission power or cost.

• Our Goal: To determine whether JT-CoMP with CLSM integration can significantly:

  • Improve average user throughput.

  • Enhance cell edge performance.

  • Increase spectral efficiency.

  • Maintain reliability for users moving at speeds up to 120 km/h.

Technical Foundations

Coordinated Multipoint (CoMP)

• A set of techniques where multiple eNodeBs (base stations) coordinate transmission/reception to minimize interference.

• Instead of treating signals from neighboring cells as interference, CoMP treats them as useful cooperative signals.

Joint Transmission (JT) CoMP

• Multiple eNodeBs transmit the same data simultaneously to a UE.

• Improves signal quality through constructive combining of transmissions.

Intra-site vs Inter-site JT-CoMP

• Intra-site JT-CoMP: Coordination between multiple sectors of the same eNB site.

  • Pros: Low overhead, easy synchronization.

  • Cons: Limited scope (doesn’t reduce inter-cell interference from neighbors).

• Inter-site JT-CoMP: Coordination across different eNB sites.

  • Pros: Stronger ICI reduction, better for fast-moving UEs.

  • Cons: Higher backhaul requirements, more complex synchronization.

Closed Loop Spatial Multiplexing (CLSM)

• MIMO transmission strategy where UE sends precoding matrix indicators (PMI) back to eNB.

• Enables layered transmission, optimizing SINR.

• Works best in low mobility conditions (accurate CSI feedback).

• Performance declines in high mobility, but CoMP can compensate.

Methodology

Tool Used: Vienna LTE-A link-level simulator (open-source simulator for LTE/5G system-level evaluation.)

Network Configuration

Scenarios Compared

• HetNet without CoMP (Baseline).

• Intra-site JT-CoMP (CoMP within one eNB).

• Inter-site JT-CoMP (CoMP across multiple eNBs).

Metrics Evaluated

1. Average UE Throughput (Mbps).

2. Cell Edge Throughput (Mbps).

3. Spectral Efficiency (bps/Hz).

Results

Average UE Throughput vs Velocity

• Trend: All schemes degrade with velocity due to Doppler effects.

• Baseline HetNet: Poor performance, fluctuates heavily.

• Intra-site JT-CoMP: Higher throughput at low speeds (≤ 30 km/h).

• Inter-site JT-CoMP: More resilient at higher speeds (≥ 60 km/h) due to better ICI mitigation.

Cell Edge Throughput vs Velocity

• Baseline HetNet: Severe degradation as UE begins moving.

• Intra-site JT-CoMP: Some improvement, but limited in mitigating neighboring cell interference.

• Inter-site JT-CoMP: Best performance, especially at cell boundaries under high mobility.

Spectral Efficiency vs Velocity

• HetNet without CoMP: Lowest efficiency, worsens at higher mobility.

• Intra-site JT-CoMP: Improvement in localized coordination.

• Inter-site JT-CoMP: Highest efficiency overall due to interference suppression and constructive combining.

Tradeoffs Observed:

• Intra-site CoMP is more practical for low-mobility deployments (urban pedestrians, IoT).

• Inter-site CoMP is essential for high-mobility environments (commuters, vehicles).

Final Thoughts

This research highlighted how Joint Transmission Coordinated Multipoint (JT-CoMP), when integrated with Closed Loop Spatial Multiplexing (CLSM), can significantly improve the reliability and efficiency of 5G heterogeneous networks under user mobility. By analyzing intra-site and inter-site CoMP across different velocities, the study demonstrated clear trade-offs: intra-site schemes excel at low mobility, while inter-site coordination is more effective at high mobility and cell edges. These findings not only advance academic understanding of interference management but also provide practical guidance for operators seeking scalable, cost-efficient strategies to deliver consistent performance in real-world 5G deployments.