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, the 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 the array signal processing point of view. Taking millimeter-wave-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.

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

Emerging communication network applications including fifth-generation (5G) cellular and the Internet-of-Things (IoT) will almost certainly require location information at as many network nodes as possible. Given the energy requirements and lack of indoor coverage of Global Positioning System (GPS), collaborative localization appears to be a powerful tool for such networks. In this paper, we survey the state of the art in collaborative localization with an eye toward 5G cellular and IoT applications. In particular, we discuss theoretical limits, algorithms, and practical challenges associated with collaborative localization based on range-based as well as range-angle-based techniques.

https://research.chalmers.se/publication/503903/file/503903_Fulltext.pdf

Collaborative Sensor Network Localization: Algorithms and Practical Issues

With the congestion of the sub-6 GHz spectrum, the interest in massive multiple-input multiple-output (MIMO) systems operating on millimeter wave spectrum grows. In order to reduce the power consumption of such massive MIMO systems, hybrid analog/digital transceivers and application of low-resolution digital-to-analog/analog-to-digital converters have been recently proposed. In this work, we investigate the energy efficiency of quantized hybrid transmitters equipped with a fully/partially connected phase-shifting network composed of active/passive phase shifters and compare it to that of quantized digital precoders. We introduce a quantized single-user MIMO system model based on an additive quantization noise approximation considering realistic power consumption and loss models to evaluate the spectral and energy efficiencies of the transmit precoding methods. Simulation results show that partially connected hybrid precoders can be more energy-efficient compared to digital precoders, while fully connected hybrid precoders exhibit poor energy efficiency in general. Also, the topology of phase-shifting components offers an energy-spectral efficiency tradeoff: Active phase shifters provide higher data rates, while passive phase shifters maintain better energy efficiency.

Energy Efficiency of mmWave Massive MIMO Precoding With Low-Resolution DACs

Millimeter-wave (mmWave) communication with multi-gigahertz bandwidth availability enables much higher capacity and transmission rate than conventional microwave communications. However, mmWave signals suffer from propagation-related shortcomings. The flexibility of UAV communication can compensate most of the deficiencies of mmWave signals, preserve its advantages, and provide more opportunities. In this article, we present the current state of the art, research opportunities, and challenges for UAV mmWave communication. We present channel characteristics and show its modeling issues. We further analyze how to achieve UAV mmWave channel acquisition and precoder design. Moreover, we discuss the challenges and possible solutions for UAV mmWave cellular networks, including UAV-to-base-station and UAV-to-user communications and spectrum sharing.

Research Challenges and Opportunities of UAV Millimeter-Wave Communications

We propose a robust beam-tracking algorithm to maintain the communication link between a base station (BS) and a mobile station (MS) in a millimeter wave mobile communications system, with antenna arrays at both the BS and MS. The channel is tracked with the extended Kalman filter (EKF) at the static BS and the beamforming weight is updated with a robust minimum mean squared error beamformer bounded by the array vector error which is fed from the error variance estimated by the EKF. Results show that our proposed method is able to maintain a link with the MS with a smaller mismatch error compared with existing beamtracking method at moderate MS mobility and antenna array size.

Robust Beam-Tracking for mmWave Mobile Communications

Analog beamforming with phased arrays is a promising technique for 5G wireless communication at millimeter wave frequencies. Using a discrete codebook consisting of multiple analog beams, each beam focuses on a certain range of angles of arrival or departure and corresponds to a set of fixed phase shifts across frequency due to practical hardware considerations. However, for sufficiently large bandwidth, the gain provided by the phased array is actually frequency dependent, which is an effect called beam squint, and this effect occurs even if the radiation pattern of the antenna elements is frequency independent. This paper examines the nature of beam squint for a uniform linear array (ULA) and analyzes its impact on codebook design as a function of the number of antennas and system bandwidth normalized by the carrier frequency. The criterion for codebook design is to guarantee that each beam's minimum gain for a range of angles and for all frequencies in the wideband system exceeds a target threshold, for example 3 dB below the array's maximum gain. Analysis and numerical examples suggest that a denser codebook is required to compensate for beam squint. For example, 54% more beams are needed compared to a codebook design that ignores beam squint for a ULA with 32 antennas operating at a carrier frequency of 73 GHz and bandwidth of 2.5 GHz. Furthermore, beam squint with this design criterion limits the bandwidth or the number of antennas of the array if the other one is fixed.

Effect of Wideband Beam Squint on Codebook Design in Phased-Array Wireless Systems

We present a tracking algorithm to maintain the communication link between a base station (BS) and a mobile station (MS) in a millimeter wave (mmWave) communication system, where antenna arrays are used for beamforming in both the BS and MS. Downlink transmission is considered, and the tracking is performed at the MS as it moves relative to the BS. Specifically, we consider the case that the MS rotates quickly due to hand movement. The algorithm estimates the angle of arrival (AoA) by using variations in the radiation pattern of the beam as a function of this angle. Numerical results show that the algorithm achieves accurate beam alignment when the MS rotates in a wide range of angular speeds. For example, the algorithm can support angular speeds up to 800 degrees per second when tracking updates are available every 10 ms.

Beampattern-Based Tracking for Millimeter Wave Communication Systems

We analyze a one-bit wireless transceiver whose architecture is simple enough that its power versus performance profile can be modeled analytically. We then utilize multiple such transceivers in a communication system operating at millimeter-wave carrier frequencies. Various aspects of the system are analyzed, including the optimum achievable throughput for a given amount of total consumed power. An analogy is drawn between the “transceiver cell” proposed herein and a “computational cell” commonly used in neural networks that allows us to apply neural-network type algorithms to aid in difficult tasks such as channel estimation for a large number of transceivers.

Power-performance analysis of a simple one-bit transceiver

We analyze the channel capacity of a system with a large number of one-bit transceivers in a classical Rayleigh environment with perfect channel information at the receiver. With M transmitters and N =αM receivers, we derive an expression of the capacity per transmitter C, where C ≤ min(1,α), as a function of a and signal-to-noise ratio (SNR) ρ, when M → ∞. We show that our expression is a good approximation for small M, and provide simple approximations of C for various ranges of α and ρ. We conclude that at high SNR, C reaches its upper limit of one only if α > 1.24. Expressions for determining when C “saturates” as a function of α and ρ are given.

Capacity of multiple one-bit transceivers in a Rayleigh environment

One-bit transceivers with strongly nonlinear characteristics are being considered for wireless communication because of their low cost and low power consumption. Although each such transceiver can support only a low data rate, multiple such transceivers can be used to obtain an aggregate high data rate. An important part of many communication systems is the process of channel estimation, which is particularly challenging when the estimation process uses these transceivers. The standard analysis of estimation mean-square error versus training length that is available for linear transceivers does not apply with the nonlinearities inherent in one-bit transceivers. We analyze the training requirements in a large- scale system and show that the optimal number of training symbols strongly depends on the number of receivers, and the optimal number of training symbols can be significantly smaller than the number of transmitters. These results contrast sharply with classical results obtained with linear transceivers.

Kang Gao

Papers

Effect of Wideband Beam Squint on Codebook Design in Phased-Array Wireless Systems

Beampattern-Based Tracking for Millimeter Wave Communication Systems

Power-performance analysis of a simple one-bit transceiver

Capacity of multiple one-bit transceivers in a Rayleigh environment

Channel Estimation with One-Bit Transceivers in a Rayleigh Environment