Hybrid MIMO Architectures for Millimeter Wave Communications: Phase Shifters or Switches? (1512.03032v1)

Abstract: Hybrid analog/digital MIMO architectures were recently proposed as an alternative for fully-digitalprecoding in millimeter wave (mmWave) wireless communication systems. This is motivated by the possible reduction in the number of RF chains and analog-to-digital converters. In these architectures, the analog processing network is usually based on variable phase shifters. In this paper, we propose hybrid architectures based on switching networks to reduce the complexity and the power consumption of the structures based on phase shifters. We define a power consumption model and use it to evaluate the energy efficiency of both structures. To estimate the complete MIMO channel, we propose an open loop compressive channel estimation technique which is independent of the hardware used in the analog processing stage. We analyze the performance of the new estimation algorithm for hybrid architectures based on phase shifters and switches. Using the estimated, we develop two algorithms for the design of the hybrid combiner based on switches and analyze the achieved spectral efficiency. Finally, we study the trade-offs between power consumption, hardware complexity, and spectral efficiency for hybrid architectures based on phase shifting networks and switching networks. Numerical results show that architectures based on switches obtain equal or better channel estimation performance to that obtained using phase shifters, while reducing hardware complexity and power consumption. For equal power consumption, all the hybrid architectures provide similar spectral efficiencies.

• The paper introduces innovative hybrid MIMO architectures that use both phase shifters and switching networks to reduce power consumption and system complexity.
• It develops a compressive sensing-based channel estimation algorithm that achieves low NMSE with reduced training overhead.
• Simulations reveal that switch-based architectures can match or exceed the spectral efficiency of phase shifter designs under equal power constraints.

Hybrid MIMO Architectures for Millimeter Wave Communications: Phase Shifters or Switches?

Millimeter wave (mmWave) frequencies are emerging as a prominent feature for fifth generation (5G) cellular networks due to their ability to support high data rates through large bandwidth channels. Hybrid analog/digital multiple-input multiple-output (MIMO) architectures are integral to mmWave systems because they reduce the number of radio frequency (RF) chains and analog-to-digital converters (ADCs) required, thus minimizing power consumption and system complexity.

This paper presents an in-depth analysis of hybrid MIMO architectures that leverage phase shifters and novel switching networks for their analog processing stages. It proposes several new hybrid configurations using switching networks, evaluates their power consumption, and develops algorithms for channel estimation and spectral efficiency optimization.

Key Contributions

1. Architectural Proposals: The paper proposes hybrid MIMO architectures using phase shifters and switching networks. Specifically:
   • Two phase shifter-based architectures: one with variable phase shifters connected to all antennas (A1) and another with phase shifters connected to antenna subsets (A2).
   • Four switching-based architectures: one with each RF chain connected to several antennas using switches (A3), another with switches connected to subsets of antennas (A4), a simple switching network where each RF chain connects to a single antenna (A5), and a variant connecting subsets of antennas (A6).
2. Power Consumption Model: A comprehensive power consumption model for mmWave MIMO receivers is developed using recent advancements in low-power circuits. The model reveals the relative power efficiency of phase-shifter and switch-based hybrid architectures under various configurations.
3. Channel Estimation Algorithms: The paper introduces a new open-loop, compressive channel estimation technique consistent with both phase shifter and switch-based hybrid architectures. This technique employs a compressive sensing framework to leverage the spatial sparsity of mmWave channels, offering robust performance even in low-SNR regions.
4. Combining Algorithms: For the proposed architectures, the paper develops algorithms to design the hybrid combining matrices. The methods include:
   • Sparse reconstruction for phase-shifting networks.
   • Hybrid antenna selection algorithms for switching networks, optimizing the use of RF chains to maximize spectral efficiency while minimizing power consumption.

Numerical Results and Analysis

The paper's numerical results validate the theoretical contributions through simulations.

• Channel Estimation Performance: The proposed compressive sensing-based method shows a normalized mean squared error (NMSE) similar to exhaustive search methods, with substantial reductions in the required training overhead. Random and deterministic training sequences tailored for specific hardware constraints are shown to improve coherence and, consequently, estimation accuracy.
• Spectral Efficiency Evaluation: Phase shifter-based architectures generally achieve higher spectral efficiencies, particularly in configurations with sufficient RF chains approximating the channel capacity closely. However, the power-efficient switch-based architectures (specifically A5 and A6) exhibit competitive spectral efficiencies for the same power budget, showcasing their practical applicability for mobile devices and power-constrained applications.
• Trade-Off Analysis: For equal power consumption, switching-based architectures (A5 and A6) achieve similar or better spectral efficiencies compared to phase shifter-based architectures. The trade-off between power consumption, hardware complexity, and spectral performance is comprehensively captured, suggesting scenarios where each architecture may be preferable.

Implications and Future Directions

The findings indicate that hybrid MIMO architectures based on switching networks offer a viable alternative to phase shifter-based designs, especially in applications where power efficiency and complexity are critical constraints.

Future developments may focus on:

• Hardware Innovations: Advanced low-power switch designs and higher precision phase shifters to further mitigate insertion losses and improve overall system performance.
• Algorithm Refinement: Enhanced channel estimation and hybrid combining algorithms that can dynamically adapt to changing environments and higher-dimensional MIMO systems.
• Integration with 5G Standards: Testbeds and field trials to integrate these architectures into full-fledged 5G networks, assessing their real-world performance and scalability.

This paper provides a foundational assessment of different hybrid MIMO architectures, presenting comprehensive solutions to optimize performance and power efficiency in mmWave communication systems, bridging an essential gap in the current state of mmWave MIMO research.