As 5G New Radio (NR) is being rolled out, research effort is being focused on the evolution of what is to come in the post-5G era. In order to meet the diverse requirements of future wireless communication in terms of increased capacity and reduced latency, technologies such as distributed massive Multiple-Input Multiple-Output (MIMO), sub-millimeter wave and Tera-hertz spectrum become technology components of interest. Furthermore, to meet the demands on connectivity anywhere at anytime, non-terrestrial satellite networks will be needed, which brings about challenges both in terms of implementation as well as deployment. Finally, scaling up massive Internet-of-Things (IoT), energy harvesting and Simultaneous Wireless Information and Power Transfer (SWIPT) is foreseen to become important enablers when deploying a large amount of small, low-power radios. In this paper, we will discuss some of the important opportunities these technologies bring, and the challenges faced by the microwave and wireless communication communities.

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Implementation Challenges and Opportunities in Beyond-5G and 6G Communication

The Internet of Things (IoT) has been a promising communication paradigm that involves sensors, microcontrollers, and transceivers for an efficient communication and computation system. The infrastructure and the applications shall enable and improve the intelligent management of our city service, workspace, and daily life. This article aims at the future 6G vision of IoT, and discusses the convergence with the Radio-over-Fiber (RoF) system. Comparing with the IoT services included in the 5G deployment, 6G IoT exploits high-density heterogeneous devices involving extremely high capacity, supporting much more robust system architecture and artificial intelligence (AI)-based smart algorithms. The RoF is one of the most promising enablers for the outstanding characters of flexibility and efficiency of 6G IoT systems. This article first introduces the IoT envolution roadmap from 5G toward 6G and the potency of optic fiber and RoF technologies. Then, we present the rapidly expanding RoF market and compatible technologies related to IoT-RoF convergence with the discussion on the current outstanding works in multiple dimensions. Finally, we investigate the challenges ahead for the future RoF supported 6G IoT system and the emerging technology solutions.

Toward 6G Internet of Things and the Convergence With RoF System

There has been significant research devoted to the development of distributed microwave wireless systems in recent years. The progression from large, single-platform wireless systems to collections of smaller, coordinated systems on separate platforms enables significant benefits for radar, remote sensing, communications, and other applications. The ultimate level of coordination between platforms is at the wavelength level, where separate platforms operate as a coherent distributed system. Wireless coherent distributed systems operate in essence as distributed phased arrays, and the signal gains that can be achieved scale proportionally to the number of transmitters squared multiplied by the number of receivers, providing potentially dramatic increases in wireless system capabilities enabled by increasing the number of nodes in the array. Coordinating the operations of nodes in a distributed array requires accurate control of the relative electrical states of the nodes. The basic challenge is the synchronization and stability of the relative phases of the signals transmitted or received. Generally, such control requires wireless frequency synchronization, phase calibration, and time alignment. For radar operations, phase control also requires high-accuracy knowledge of the relative positions of the nodes in the array to support beamforming. Various technologies have been developed in recent years to address the coordination challenges for closed-loop applications, such as distributed communications, and more recently, there has been growing interest in new technologies for open-loop applications, such as radar and remote sensing. This article presents an overview of distributed phased arrays, the principal challenges involved in their coordination, and recent research progress addressing these challenges.

Distributed Phased Arrays: Challenges and Recent Advances

Distributed multiple-input-multiple-output (D-MIMO) is a very promising technique to improve capacity and quality of service for emerging 5G systems. In this paper, we propose a flexible and low-cost testbed for D-MIMO systems, where a sigma–delta over fiber (SDoF) solution is proposed to address the fundamental challenge of phase-coherent transmission between multiple distributed access points (APs) from a central unit (CU). Employing low-cost standardized optical data interconnects, SDoF, brings a very flexible and software controlled transmission of RF signals over optical fiber. An SDoF-based D-MIMO testbed with a CU feeding 12 distributed APs has been realized and experimentally evaluated. Initial experiments of D-MIMO and conventional colocated MIMO systems at 2.365 GHz have been performed to demonstrate the flexibility and potential of the proposed testbed as a new powerful tool in the design and analysis of emerging communication system concepts.

A Low-Complexity Distributed-MIMO Testbed Based on High-Speed Sigma–Delta-Over-Fiber

We present an open-loop microwave distributed beamforming system using a self-mixing circuit for wireless frequency synchronization between two transmitting nodes in relative motion. Distributed beamforming on arrays of wireless nodes requires accurate coordination of the electrical states of each node to ensure that transmitted signals arrive at the desired destination with sufficient phase alignment for high signal gain. Of the necessary coordination parameters, synchronizing the frequencies of the internal oscillators is among the most crucial to ensure distributed coherence. In this work, we demonstrate distributed beamforming between separate microwave wireless transmitters using wireless frequency synchronization in a hierarchical coordination topology that is directly scalable to larger array sizes. The primary node transmits a spectrally sparse two-tone waveform that is received by a self-mixing circuit at the secondary node. A 10-MHz frequency reference of the primary node is modulated onto the tone separation of the waveform; upon self-mixing, the demodulated 10-MHz frequency reference is passed to a phase-locked loop to discipline the oscillator of the secondary node. We demonstrate beamforming at a 1.5-GHz carrier frequency between the two nodes, performed while moving the primary node, demonstrating the ability to maintain greater than 90% ideal distributed beamforming gain.

Open-Loop Distributed Beamforming Using Wireless Frequency Synchronization

State-of-the-art high-speed multi-mode vertical-cavity surface-emitting-lasers (VCSELs) have a 3-dB analog bandwidth of $\sim$ 25 GHz and can operate up to 55 Gbps error-free, without need of equalization electronics. This makes them strong candidates as optical sources for ultra-high speed sigma-delta-over-fiber (SDoF) communication links. This paper examines the effects of VCSEL characteristics on a 32-Gbps SDoF communication link with a 50 m, OM4, multimode optical fiber. Three high-speed, 850-nm-wavelength VCSELs with oxide aperture sizes of 8.3, 6.6, and 4.6  $\mu$ m were tested. The effects of different modulation voltage amplitudes and bias currents were also investigated. The link performance and VCSEL power consumption were used as figures-of-merits. By avoiding sub-laser threshold modulation for the VCSEL, each of three VCSELs could provide similar link performance. We show that, compared to conventional datacom links, eased bit-error-rate requirements enabled by bi-level nature of sigma-delta modulation makes it possible to use significantly lower VCSEL bias currents. This in turn enables a reduced VCSEL power consumption (50% lower heat-to-bit ratio) and a potential longer VCSEL lifespan. For a SDoF modulated, 160-Mbaud, QAM64 signal centered at 12 GHz an error vector magnitude of $-$ 28 dB was achieved with the VCSEL having oxide aperture size of 4.6  $\mu$ m operating at only 2 mA bias current and 0.25 V modulation amplitude. This results in a heat-to-bit ratio of only 0.1 mW/Gb/s (or equivalently 0.1 pJ/b), and a current density in the VCSEL that is less than 10 kA/cm $^2$ .

Effect of VCSEL Characteristics on Ultra-High Speed Sigma-Delta-Over-Fiber Communication Links

Distributing the antennas of a base station is a method to increase the coverage and the capacity of a conventional, co-located MIMO communication system. However, physically separated antennas/access points (AP) creates a fundamental challenge of RF phase synchronization required for joint/coordinated transmission in distributed MIMO (D-MIMO) systems. In this paper we compare co-located-and distributed-MIMO wireless communication thru various measurements using a fully synchronized, low-complexity, twelve-channel testbed based on sigma-delta-over-fiber (SDoF). Single and multi-user measurements are performed in a laboratory environment and the error-vector-magnitude (EVM), the received power and signal-to-interference ratio (SIR) is calculated. We show that distributing the access points improves the received symbol EVM up to 1.6-dB and increases the coverage of the service area by delivering more uniform received power levels.

Evaluation of Distributed MIMO Communication Using a Low-Complexity Sigma-Delta-over-Fiber Testbed

Sigma-delta-over-fiber (SDoF) using high speed-low cost optical components for datacom enables high speed transmission at X-band. This paper examines the limitations of SDoF in terms of maximum frequencies and highest bit rates. Different modulation formats, symbol rates and carrier frequencies are investigated. A high speed 850 nm vertical-cavity surface-emiting-laser (VCSEL) is used as an optical source to transmit the optical signal over a 50 meter, OM4, multimode fiber (MMF). Error vector magnitude (EVM) less than 6% is achieved at a 12-GHz center frequency for a 8-Gbps QAM256 modulated signal.

A Flexible Multi-Gbps Transmitter Using Ultra-High Speed Sigma- Delta-over- Fiber

Radio-over-fiber is a popular technique to establish communication links between a central location and many remote antenna units. Many different architectures are available for the downlink, i.e., for the communication link from the central unit to the remote antennas. On the contrary, the low-cost and low-complexity requirement of the remote units makes it difficult to devise architectures suitable for the uplink, i.e., for the communication link from the remote antennas to the central unit. In this article, we address this and propose a low-complexity, all-digital, time-division-duplex communication architecture. For the downlink, a band-pass sigma-delta-over-fiber is employed. In the receive mode, the uplink includes an all-digital pulse-width-modulation technique. The received radio frequency (RF) signal is quantized into a binary stream through comparison with a tailored reference signal provided by the central unit. The direct quantization of the RF signal eliminates any need for local-oscillator and mixer stages at the remote units. The performance of the proposed architecture is investigated through extensive simulations and measurements. For instance, the all-digital, time-division duplex communication link provides −30.0 dB and −25.5 dB normalized mean square error signal quality through downlink and uplink communication with 20-MHz, 64-quadrature amplitude modulation signals centered at 2.365-GHz, respectively.

Ibrahim Can Sezgin

Papers

A Low-Complexity Distributed-MIMO Testbed Based on High-Speed Sigma–Delta-Over-Fiber

Effect of VCSEL Characteristics on Ultra-High Speed Sigma-Delta-Over-Fiber Communication Links

Evaluation of Distributed MIMO Communication Using a Low-Complexity Sigma-Delta-over-Fiber Testbed

A Flexible Multi-Gbps Transmitter Using Ultra-High Speed Sigma- Delta-over- Fiber

All-Digital, Radio-Over-Fiber, Communication Link Architecture for Time-Division Duplex Distributed Antenna Systems