Remote Access Unit Research Articles

Reconfigurable Radio-Over-Fiber Networks [Invited

This paper discusses the advantages of reconfigurable radio-over-fiber (RoF) networks and presents key activities in coherent remote access units, silicon photonics for microwave photonics, optical switching, and photonically reconfigurable antennas. These research areas enable functionalities that improve the utilization, flexibility, and performance of RoF networks.

Multi-service RoF links with colorless upstream transmission based on orthogonal phase-correlated modulation.

We propose and experimentally demonstrate a full-duplex radio-over-fiber (RoF) system with colorless upstream transmission based on orthogonal phase-correlated modulation (OPM). This new OPM scheme, which realized by a polarization rotator (PR) and a single-driver Mach-Zahnder modulator (MZM) at the central office (CO), achieves polarization-orthogonality between the optical carrier (OC) and subcarriers generated by radio frequency (RF) signals. By adjusting a polarization controller (PC) in the remote access units (RAU), different modulation schemes can be flexibly implemented, e.g. double-sideband (DSB) modulation for low RF service and optical carrier suppression (OCS) modulation for millimeter-wave (mm-wave) service. In the meantime, the OC can be reused for the upstream transmission without any filtering and additional PC. A proof-of-concept experiment is conducted to demonstrate the feasibility of proposed scheme, where downstream 800-Mb/s orthogonal frequency division multiplexing (OFDM) signal at 58 GHz as an mm-wave service and 800-Mb/s OFDM signal at 0.3 GHz as a low frequency wireless service, as well as an upstream 1-Gb/s on-off keying (OOK) are simultaneously delivered in a shared architecture. By providing heterogeneous services and colorless upstream transmission, the proposed architecture can be seamlessly integrated in wavelength division multiplexing passive optical network (WDM-PON).

Photonic-aided pre-coding QAM signal transmission in multi-antenna radio over fiber system

A novel method has been proposed and experimentally demonstrated to provide photonic-aided pre-coding quadrature amplitude modulation (QAM) signal transmission in multi-antenna radio over fiber system to increase the throughput. For downlink, two different multi-level amplitude-shift-keying (M-ASK) modulated signals, such as 4-ASK signals, are applied on two uncorrelated optical Mach–Zehnder modulators at central office. After photonic-aided pre-coding module and photo-detection process, the received M-ASK mm-waves from two remote access units (RAUs) can be synthesized to a M2-QAM signal in the proposed system. Regardless of forward error correction (FEC) coding overhead, the 4-Gb/s and 8-Gb/s 16-QAM mm-wave signals are obtained from two independent 2-Gb/s and 4-Gb/s 4-ASK 40-GHz channels, respectively. The experimental results show that a doubled bit rate of the original 4-ASK one can be achieved without additional digital signal processing (DSP) in small cell RAUs and mobile users.

Downlink massive distributed antenna systems scheduling

This study investigates the scheduling problem for a single-cell downlink distributed antenna systems (DASs) with a massive number of remote access units (RAUs). To reduce signalling overhead under limited backhaul capacity, the authors make use of local long-term channel state information (CSI) in coordinated scheduling design. They first derive the ergodic rate expressions for both the single RAU transmission and the cooperative RAU transmission modes as functions of long-term CSI. Then greedy scheduling algorithms (GSAs) aiming for the maximum ergodic sum rate for the massive DAS using long-term CSI are proposed. To mitigate the intra-cell interference, a two-stage GSA with hybrid transmission mode is devised. Asymptotic analysis reveals that as the number of RAUs goes to infinity, intra-cell interference can be effectively mitigated. Simulation results verify the analysis and demonstrate that the two-stage GSA exhibits a higher ergodic sum-rate.

Photonic Precoding for Millimeter-Wave Multicell MIMO in Centralized RoF System

We propose a centralized photonic precoding method for millimeter-wave (MMW) multicell multiple-input multiple-output (MIMO) wireless communications. By applying the multicell MIMO and hence exploiting the spatial multiplexing gain over MMW, the system capacity is enhanced compared with the conventional single-input single-output (SISO) system. Based on radio-over-fiber (RoF) techniques, the multicell function is realized in a centralized location, which simplifies the design of remote access units. Traversing over the optical links enabled by the RoF infrastructure, we implement a novel method of photonic precoding to realize the zero-forcing precoding approach, which simplifies the user equipment complexity. A two-cell MIMO downlink transmission using the proposed method is experimentally demonstrated. Our results show that original signals can be directly recovered at the receivers with a power penalty <;1 dB compared with SISO transmission. The effect of varying photonic parameters is also analyzed.

Adaptive Photonics-Aided Coordinated Multipoint Transmissions for Next-Generation Mobile Fronthaul

We propose and demonstrate a concept of photonics-aided coordination for multi-cell downlink transmissions. The concept targets reliable high-throughput applications for future-proof millimeter-wave small cells based on radio-over-fiber fronthaul. Based on the physical properties of millimeter waves and the phase and magnitude relationship during optoelectrical conversion, a photonics-aided coordination module is proposed, which adjusts the overall channel matrix of optical wireless transmissions in order to achieve a distortion-free reception. Exploiting the multitude and density of remote access units, we specifically propose three methods of coordination, namely, photonics-aided amplification, synthesis, and interference cancellation, which directly enhance the signal-to-noise ratio, increase the throughput, and mitigate the interference of cell-edge users, respectively. These methods are realized at the physical layer and can be transparent to data, which reduces the coordination overhead. Prototypes, involving end-to-end downlink transmissions, are established, which demonstrate the feasibility and effectiveness of the proposed methods for both scalar and vector signals.

Energy-Efficient Resource Allocation in OFDM Systems With Distributed Antennas

In this paper, we develop an energy-efficient resource-allocation scheme with proportional fairness for downlink multiuser orthogonal frequency-division multiplexing (OFDM) systems with distributed antennas. Our aim is to maximize energy efficiency (EE) under the constraints of the overall transmit power of each remote access unit (RAU), proportional fairness data rates, and bit error rates (BERs). Because of the nonconvex nature of the optimization problem, obtaining the optimal solution is extremely computationally complex. Therefore, we develop a low-complexity suboptimal algorithm, which separates subcarrier allocation and power allocation. For the low-complexity algorithm, we first allocate subcarriers by assuming equal power distribution. Then, by exploiting the properties of fractional programming, we transform the nonconvex optimization problem in fractional form into an equivalent optimization problem in subtractive form, which includes a tractable solution. Next, an optimal energy-efficient power-allocation algorithm is developed to maximize EE while maintaining proportional fairness. Through computer simulation, we demonstrate the effectiveness of the proposed low-complexity algorithm and illustrate the fundamental tradeoff between energy- and spectral-efficient transmission designs.

Client-Weighted Medium-Transparent MAC Protocol for User-Centric Fairness in 60 GHz Radio-Over-Fiber WLANs

We present a novel client-weighted medium-transparent medium access control (CW-MT-MAC) protocol with enhanced fairness service delivery properties accompanied by a low-loss remote access unit (RAU) architecture for use in indoor, Gbps-capable, 60&#xA0;GHz radio-over-fiber (RoF) wireless local area networks (WLANs). Our approach relies on incorporating a client-weighted algorithm (CWA) in the optical capacity allocation mechanism employed in the MT-MAC scheme, so as to distribute the available wavelengths to the different antenna units according to the total number of active users served by each individual antenna. The protocol&#x2019;s throughput and delay fairness characteristics are evaluated and validated through both simulations and analytic modeling for saturated network traffic operational conditions. In addition, extended simulation-based performance analysis for nonsaturated network conditions and for different end-user distributions, traffic loads, and available optical wavelengths at 1&#xA0;Gbps data rates is presented. Our results confirm that complete throughput equalization can be achieved even for highly varying user population patterns when certain wavelength availability conditions are satisfied. At the same time, the presented scheme manages to equalize the average packet delays amongst packets generated by all RAUs while concurrently dropping the packet delay variation metric that is essential for quality of service delivery. Finally the proposed RAU design reduces insertion losses by almost 14&#xA0;dB compared to RAU elements used in MT-MAC-compatible bus networks, extending in this way the number of supported RAUs by an order of magnitude and enabling the formation of extended-reach, high-speed RoF WLANs.

Energy- and Spectral-Efficiency Tradeoff for Distributed Antenna Systems with Proportional Fairness

Energy efficiency(EE) has caught more and more attention in future wireless communications due to steadily rising energy costs and environmental concerns. In this paper, we propose an EE scheme with proportional fairness for the downlink multiuser distributed antenna systems (DAS). Our aim is to maximize EE, subject to constraints on overall transmit power of each remote access unit (RAU), bit-error rate (BER), and proportional data rates. We exploit multi-criteria optimization method to systematically investigate the relationship between EE and spectral efficiency (SE). Using the weighted sum method, we first convert the multi-criteria optimization problem, which is extremely complex, into a simpler single objective optimization problem. Then an optimal algorithm is developed to allocate the available power to balance the tradeoff between EE and SE. We also demonstrate the effectiveness of the proposed scheme and illustrate the fundamental tradeoff between energy- and spectral-efficient transmission through computer simulation.

Research on Radio Frequency Assignment Mechanism of the RoFSO Network Based on Site-Specific Cognition and Utility Function

For the problem of the throughput degradation and fairness deterioration of the conventional radio frequency (RF) allocation mechanism in “hot point” areas, this paper presented a novel RF allocation mechanism using radio over free space optics (RoFSO) technology. The proposed algorithm contained initial RF assignment and optimal RF assignment. The initial RF allocation was built on the distribution of adjacent remote access units (RAUs). In addition, the optimal RF allocation includes RF switching and RF scheduling, both of which were designed based on site-specific cognition of stations. Through the analysis of simulation results, we know that our proposed algorithm in RoFSO network outperform the conventional RF allocation algorithm in wireless local area network (WLAN) by 350%, 45%, and 90% for average goodput of each user, fairness index of entire network and average frame service delay (FSD), respectively.

Dynamically reconfigurable radio-over-fiber network with medium access control protocol to provide network access to train passengers

An innovative dynamically reconfigurable radio-over-fiber (RoF) network equipped with an intelligent medium access control (MAC) protocol is proposed to provide broadband access to train passengers in railway high-speed mobile applications. The proposed RoF network architecture is based on a reconfigurable control station and remote access unit (RAU) that is equipped with a fixed filter and tunable filter. The proposed hybrid frequency-division multiplexing/time division multiple access (FDM/TDMA) based MAC protocol realizes failure detection/recovery and dynamic wavelength allocation to remote access units. Simulation result shows that with the proposed MAC protocol, the control station can detect failures and recover and dynamic wavelength allocation can increase the wavelength resource utilization to maintain network performance.

Linearization and dispersion tolerable technique for remote access unit in uplink RoF system based on gain-saturated RSOA cascaded EAM

A novel scheme of a remote access unit which is based on an electroabsorption modulator cascaded by a gain-saturated reflective semiconductor optical amplifier is proposed to improve a transmission performance of uplink signal. The cascaded gain-saturated reflective semiconductor optical amplifier plays a role in enhancing linearity of the electroabsorption modulator as well as suppressing dispersion-induced carrier suppression in the uplink transmission using its nonlinear gain property. In the proposed scheme, carrier-to-interference ratio of transmitted 10-GHz band uplink signal was improved by 10.7 dB and the dispersion-induced carrier suppression of 5.65-GHz RF carrier was greatly mitigated by 33.4 dB through the 75.6-km optical link.

Frequency up‐conversion in multimode fiber‐fed broadband wireless networks by using agile tunable laser source

AbstractDelivery of high‐frequency carriers to remote radio access units, which are fed by multimode fibers, is demonstrated with an agile tunable laser for the first time. The phase section of the tunable laser is modulated with a low‐frequency electrical signal resulting in an up‐conversion factor greater than 20. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 28–30, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20035

Wireless in Local Loop: Some Key Issues

Wireless links are emerging as a superior alternative to the copper pair for the local loop in telephony. However, to provide for sufficient subscriber density with a reasonable bandwidth allocation, a micro-cellular architecture is a must. Further, the wireless local loop system is best interfaced as a remote access unit to an exchange. The V5.2 standard provides a convenient protocol for interfacing in this manner to all exchanges.