Spatial- and Frequency-Wideband Effects in Millimeter-Wave Massive MIMO Systems (1708.07605v5)

Abstract: When there are a large number of antennas in massive MIMO systems, the transmitted wideband signal will be sensitive to the physical propagation delay of electromagnetic waves across the large array aperture, which is called the spatial-wideband effect. In this scenario, transceiver design is different from most of the existing works, which presume that the bandwidth of the transmitted signals is not that wide, ignore the spatial-wideband effect, and only address the frequency selectivity. In this paper, we investigate spatial- and frequency-wideband effects, called dual-wideband effects, in massive MIMO systems from array signal processing point of view. Taking mmWave-band communications as an example, we describe the transmission process to address the dual-wideband effects. By exploiting the channel sparsity in the angle domain and the delay domain, we develop the efficient uplink and downlink channel estimation strategies that require much less amount of training overhead and cause no pilot contamination. Thanks to the array signal processing techniques, the proposed channel estimation is suitable for both TDD and FDD massive MIMO systems. Numerical examples demonstrate that the proposed transmission design for massive MIMO systems can effectively deal with the dual-wideband effects.

• The paper introduces an advanced dual-wideband channel model that captures spatial and frequency selectivity to address beam squint in mmWave massive MIMO systems.
• The paper proposes innovative channel estimation techniques that reduce pilot contamination and training overhead using angular-delay orthogonality.
• Numerical simulations validate the model and methods, demonstrating significant improvements in spectral efficiency and system reliability.

Spatial- and Frequency-Wideband Effects in Millimeter-Wave Massive MIMO Systems

The paper presents an in-depth investigation into the complex interplay of spatial and frequency effects in Millimeter-Wave (mmWave) Massive Multiple-Input Multiple-Output (MIMO) systems, referred to as dual-wideband effects. The authors develop a nuanced transmission model that diverges from conventional approaches, which frequently overlook these dual-wideband considerations. This paper is particularly focused on channel estimation strategies for massive MIMO systems that include both spatial-wideband and frequency-wideband effects, building upon the basic principles of array signal processing.

Core Contributions

1. Spatial-Wideband Effect Examination: The paper commences by revisiting a foundational concept—the spatial-wideband effect—where a significant time delay impacts massive MIMO systems due to the propagation delay across a large antenna array. This is in contrast to small-scale MIMO systems where such effects are negligible. The spatial-wideband effect manifests as beam squint, highlighting the necessity for reconsidering conventional MIMO channel models.
2. Dual-Wideband Channel Modeling: The authors propose an advanced channel modeling technique that captures dual-wideband effects, namely spatial selectivity and frequency selectivity. This model incorporates the concept of beam squint, providing a framework that accurately represents mmWave band communications, advantageous for both Time Division Duplex (TDD) and Frequency Division Duplex (FDD) systems.
3. Efficient Channel Estimation Methods: With the dual-wideband channel model established, the paper outlines novel strategies for uplink and downlink channel estimation that minimize training overhead and eliminate pilot contamination. The authors leverage angular-delay orthogonality inherent in the proposed model to devise a pilot design that enables simultaneous non-interfering communication for multiple users.
4. Theoretical Insights and Performance Analysis: The paper offers substantial theoretical analysis, including proofs for the asymptotic characteristics of the dual-wideband channels. It delineates the requirements for cyclic prefix length in Orthogonal Frequency Division Multiplexing (OFDM) to effectively counteract both channel path delays and spatial-wideband impacts.
5. Numerical Validation: Extensive numerical simulations validate the channel estimation methodologies. The results corroborate that incorporating dual-wideband effects significantly improves channel estimation accuracy, demonstrating the inadequacy of conventional narrowband assumption-based methods under typical mmWave bandwidth and antenna array dimensions.

Implications and Future Directions

This paper provides a critical framework for enhancing channel estimation in massive MIMO systems by addressing dual-wideband effects—an aspect previously misrepresented or ignored in traditional models. The ability to refine signal processing techniques with consideration of these effects heralds potential improvements in spectral and energy efficiency in next-generation wireless systems.

Practically, the results could catalyze more advanced beamforming techniques that exploit spatial and frequency diversities, leading to enhanced data rates and reliability in dense urban environments. Theoretical implications extend to the necessity of revisiting many established signal processing techniques that presuppose narrowband conditions.

Future research may build on this foundation to refine these methodologies for adaptive systems capable of real-time dynamic channel adaptation. Investigating robust designs against practical impairments, such as channel estimation errors or hardware non-idealities, is a natural progression of this work, ultimately paving the way for the broader deployment and commercialization of massive MIMO technologies in the mmWave spectrum.