Yizhuo Yang

Fiber-Wireless Networks and Subsystem Technologies

Hybrid fiber-wireless networks incorporating WDM technology for fixed wireless access operating in the sub-millimeter-wave and millimeter-wave (mm-wave) frequency regions are being actively pursued to provide untethered connectivity for ultrahigh bandwidth communications. The architecture of such radio networks requires a large number of antenna base-stations with high throughput to be deployed to maximize the geographical coverage with the main switching and routing functionalities located in a centralized location. The transportation of mm-wave wireless signals within the hybrid network is subject to several impairments including low opto-electronic conversion efficiency, fiber chromatic dispersion and also degradation due to nonlinearities along the link. One of the major technical challenges in implementing such networks lies in the mitigation of these various optical impairments that the wireless signals experience within the hybrid network. In this paper, we present an overview of different techniques to optically transport mm-wave wireless signals and to overcome impairments associated with the transport of the wireless signals. We also review the different designs of subsystems for integrating fiber-wireless technology onto existing optical infrastructure.

Digitized RF transmission over fiber

Our studies have shown that a digitized radio- over-fiber system based on bandpass sampling with an analog-to-digital converter having a resolution of 8 b is sufficient to construct a high-performance, cost-effective link that can allow easy integration of wireless backhaul with optical communication infrastructure. We also show that the performance of the digitized system is predominantly limited by the performance of the analog-to-digital converter used, with its quantization noise limiting the performance for the lower values of resolution (<6 b) and its jitter limiting the performance at the higher resolutions (>7 b).

Investigation on Transport Schemes for Efficient High-Frequency Broadband OFDM Transmission in Fibre-Wireless Links

This paper investigates the different transport schemes for delivery of high-frequency broadband OFDM signals via fibre-wireless links. We have investigated analog RoF, digitized RoF, and sampled RoF transport schemes and experimentally demonstrated the transmission of broadband RF-OFDM signal using these schemes. Analog RoF results in the simplest BS/RRH configuration but it demands for large optical bandwidth. The performance of this transport scheme is limited by inter-modulation distortion (IMD) and RF power fading. For the digitized RoF transport scheme based on bandpass sampling, although digital optical link is used for signal delivery, the performance is limited by signal clipping and ADC resolution. The sampled RoF transport scheme requires only a narrow optical bandwidth, but the performance is limited by IMD and noise processes introduced by bandpass sampling. Based on our analysis and experimental results, a design guide of fibre-wireless integration system for broadband RF-OFDM transmission is provided. It is found that: analog RoF is best suitable for short-range transmission; digitized RoF has the best performance; and sampled RoF offers a low-cost solution.

Multichannel Digitized RF-Over-Fiber Transmission Based on Bandpass Sampling and FPGA

In this paper, we proposed multichannel digitized RF-over-fiber (DRoF) transmission scheme and realized frequency conversion using a single analog-to-digital converter (ADC) and a single digital-to-analog converter (DAC). We gave a comprehensively analysis on this scheme, and compared it with analog links in terms of signal-to-noise ratio (SNR), dynamic range and performance degradation with channel number increasing. We experimentally implemented a multichannel DRoF uplink using commercially-available field programmable gate array (FPGA) and ADC with satisfactory error-vector-magnitudes (EVM) of less than 1%. Results show that the interference level in DRoF link is dependent upon the channel spacing of the reconstructed signal. We theoretically and experimentally proved that the DRoF technique is sufficient to construct a high-performance cost-effective RF-over-fiber link, especially for long-haul fiber radio networks.

Comparison of energy consumption of integrated optical-wireless access networks

A comparison of energy consumption of an integrated optical-wireless network with different wireless signal transport schemes is investigated. The results indicate that digitized RF/IF-over-fiber schemes are the most energy-efficient for integrated optical-wireless access networks while baseband-over-fiber scheme consumes the most energy.

Experimental Demonstration of Multi-Service Hybrid Fiber-Radio System Using Digitized RF-Over-Fiber Technique

An efficient multi-channel hybrid fiber-radio scheme for both down and up-link paths is demonstrated based on digitized RF-over-fiber technique, where the frequency translation stages are implemented using a single analog-to-digital converter (ADC) and digital-to-analog converter (DAC) instead of mixers and local oscillators. The simultaneous transmission of GSM, UMTS and WiMAX services is demonstrated with error-free reception. With a minimum sampling rate of 50 MHz and an overall optical data rate of 400 Mb/s, all the three wireless services can be successfully recovered in a full-duplex transmission. In the downlink direction, the performance is dependent on the amount of amplification required for the wireless signal at the desired carrier frequency. The amount of amplification on the other hand is dependent on the location of the wireless signal within the Nyquist zone which increases significantly with the order of the Nyquist zone. In the uplink direction, the performance is limited by inter-channel interference as a result of spectra aliasing of the regenerated IF signals which limits the signal-to-noise-ratio. With proper system design that optimizes downlink amplification and minimizes inter-channel interference, simultaneous multi-service transmission using digitized RF-over-fiber technique is able to improve spectral efficiency and minimize hardware count.

Radio-over-Fiber systems

Radio-over-Fiber (RoF) systems have been actively researched to provide future wireless broadband services. In this paper we review the schemes and strategies that have been carried out in Radio-over-Fiber networks over the years to realize high-performance RoF links.

Radio-over-fiber as the energy efficient backhaul option for mobile base stations

The power consumption of mobile base stations remains the biggest contributor in the telecommunication networks. RoF has been recognized as a solution to realize energy-efficient base station architecture. In this paper, we investigate the base station power consumption per square meter for five different RoF transmission schemes. Results show that: (1) baseband-over-fiber transmission scheme is the most energy inefficient scheme in both rural and urban areas; (2) for rural areas using macro base stations (transmitting power above 40 dBm), digitized RF/IF-over-fiber technique have advantage in terms of energy consumption; (3) for urban areas using micro base stations (transmitting power less than 25 dBm), analog RF/IF-over-fiber are the most energy-efficient schemes.

Converged fiber-wireless access networks for next generation mobile backhaul enabling CoMP

We propose a converged fiber-wireless access network based on radio-over-fiber technologies as future mobile backhaul incorporating base station cooperation schemes (CoMP). Both analog and digitized RoF technologies are discussed as possible solutions, in terms of base station configuration, system performance and energy efficiency. The implementation approach and the capacity gain of CoMP are analyzed. To reduce the processing overhead, CoMP should only be enabled with a significant CoMP gain. Experimental demonstration of the proposed scheme is presented with more than 2-dB SNR improvement.

DRoF incorporating multi-level modulation for radio-over-fiber

Digitized RF-over-fiber (DRoF) has been proposed as a high-performance low-cost solution for wireless signal distribution over optical fiber. However, the large overall optical data rate limits the implementation of DRoF for high frequency RF signals. Here, we propose a DRoF link using multi-level optical transmission to reduce the overall optical data rate and relax the requirements on the optoelectronic hardware while maintaining a relatively simplified architecture of base station. In this paper, a proof-of-concept experiment incorporating DRoF transport scheme with multi-level modulation is demonstrated. The results show that: (1) the fiber length has minimal impact on the performance of multi-level DRoF link; (2) optical links using RZ format have better performance comparing to using NRZ format; (3) an optical SNR above 11.5 dB results in a DRoF SNR of 20 dB; (4) a four-level modulation using RZ format is a promising solution to implement digitized RF-over-fiber transmission.