On the Spectral Efficiency of Full-Duplex Small Cell Wireless Systems

Abstract: We investigate the spectral efficiency of full-duplex small cell wireless systems, in which a full-duplex capable base station (BS) is designed to send/receive data to/from multiple halfduplex users on the same system resources. The major hurdle for designing such systems is due to the self-interference at the BS and co-channel interference among users. Hence, we consider a joint beamformer design to maximize the spectral efficiency subject to certain power constraints. The design problem is first formulated as a rank-constrained optimization one, and the rank relaxation method is then applied. However the relaxed problem is still nonconvex, and thus optimal solutions are hard to find. Herein, we propose two provably convergent algorithms to obtain suboptimal solutions. Based on the concept of the difference of convex functions programming, we approximate the design problem by a determinant maximization program in each iteration of the first algorithm. The second method is built upon the sequential parametric convex approximation method, which allows us to transform the relaxed problem into a semidefinite program in each iteration. Extensive numerical experiments under small cell setups illustrate that the full-duplex system with the proposed algorithms can achieve a large gain over the half-duplex one.

• The paper introduces iterative MAXDET and SDP-based algorithms to optimize beamforming for enhanced spectral efficiency.
• It demonstrates that reducing self-interference by 75–83 dB can yield up to 55% efficiency gains over half-duplex systems.
• The findings underscore the importance of advanced SI cancellation techniques for practical full-duplex small cell deployments.

Spectral Efficiency in Full-Duplex Small Cell Wireless Systems

The paper "On the Spectral Efficiency of Full-Duplex Small Cell Wireless Systems" investigates the spectral efficiency (SE) of full-duplex small cell networks. Full-duplex communication, which allows simultaneous transmission and reception on the same frequency band, has been theoretically recognized for its potential in doubling the capacity of wireless systems compared to conventional half-duplex systems. However, practical implementation is challenged by self-interference (SI) at the base station (BS) and co-channel interference (CCI) among users.

Analytical Approach

The authors focus on a scenario where a full-duplex-enabled BS communicates with multiple half-duplex users. They present an optimization problem aimed at maximizing SE under power constraints using joint beamformer design. Initially, the problem is posed as a nonconvex rank-constrained optimization problem. To address its complexity, the authors employ a rank relaxation approach, leading to a suboptimal, but tractable, problem formulation. The paper introduces two iterative algorithms to solve the relaxed problem:

1. An Iterative MAXDET-based Algorithm: This algorithm leverages the Frank-Wolfe method to approximate the design problem via determinant maximization in each iteration. It utilizes an affine majorization approach for the nonconvex term in the objective function, allowing iterative enhancement of the spectral efficiency.
2. An Iterative SDP-based Algorithm: Built upon the sequential parametric convex approximation method, this algorithm transforms the problem into a semidefinite program at each step. The algorithm enables flexible solver choice, making it computationally efficient.

Numerical Findings

The numerical experiments, conducted with configurations aligned with 3GPP LTE small cell specifications, demonstrate that full-duplex systems outperform their half-duplex counterparts in SE, contingent upon effective SI cancellation. Specifically, when SI power is reduced by more than 75–83 dB post-cancellation, the full-duplex system shows SE gains of up to 55%. The study indicates the critical requirement for effective SI mitigation to harness the potential of full-duplex systems.

Implications and Future Directions

This research adds significant value to the domain of wireless communications by showing the feasible SE gains offered by full-duplex technology, provided that current SI cancellation technologies improve. The findings suggest that full-duplex systems are particularly promising for small cell deployments due to their low transmit power and short distances.

However, the paper points out several challenges and opportunities for future research:

• Hardware Improvements: Further advances in SI cancellation hardware are necessary to make full-duplex systems viable at scale.
• Distributed and Multi-Cell Designs: Algorithms for distributed beamforming and multi-cell networks can enhance SE further by reducing CCI and leveraging the multi-user diversity inherent in larger networks.
• User Scheduling and Fairness: Future work could involve optimal scheduling and fairness considerations to ensure balanced SE across users in dual-channel systems.
• MAC Protocol Redesign: The full-duplex operation introduces new challenges and opportunities in MAC layer protocol designs, potentially enabling innovative allocation of resources.

Overall, the paper effectively demonstrates that full-duplex systems have the potential to revolutionize small cell networks' capacity, contingent on overcoming SI and CCI challenges efficiently. It paves the path for further theoretical and experimental studies aimed at realizing full-duplex technologies in practical wireless communications systems.