Prasanna A. Gamage

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 Radio-Over-Fiber Technologies for Converged Optical Wireless Access Network

With the rapid deployment of optical access networks and the growing availability of mature and cost-effective opto-electronic system technologies, a unified optical feeder network could provide seamless integration of both broadband optical and wireless access networks. Radio-over-fiber transmission has been studied extensively as a way of achieving such high levels of network integration as well as simplifying wireless base-stations. Digitized radio-over-fiber has recently been reported as a possible cost-effective pathway for achieving this by taking advantage of parallel developments in electronic sampling systems and signal processing. In this paper, we present a review of recent progress in key systems and network technologies that can support the distribution of multi-band wireless signals in a converged optical wireless access network.

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).

Design and Analysis of Digitized RF-Over-Fiber Links

In this paper, we present a performance analysis of microwave signal transmission by deploying digitized RF transport over a standard optical fiber link. The bandpass sampling technique is used to digitize the RF signal. An analytical model, which is transparent to any modulation format, was developed to assess the transmission performance of a digitized RF signal in M-level quadrature amplitude modulation (M-QAM) format. Using this analytical model, key design parameters such as bit resolution, link bit rate, receiver sensitivity, and link signal-to-noise ratio (SNR) were analyzed for a digitized 2.475 GHz RF signal with 16-QAM data modulation format and 6 MSymbols/s baud rate over a 20-km optical link. The analytical results show that the link SNR is predominantly determined by the performance of the analog-to-digital converter (ADC). We experimentally demonstrated the digitized RF transmission over a 20-km optical link. The analytical model and experimental results show that the performance of the link SNR is governed by jitter noise beyond 7 bits of resolution of the ADC. Using an ADC with 8-bit resolution, a 2.475 GHz RF signal with 16-QAM 6 MSymbols/s data was successfully transmitted and recovered with very good error vector magnitude of 1.77.

Radio-Over-Fiber Technologies for Emerging Wireless Systems

Radio-over-fiber transmission has extensively been studied as a means to realizing a fiber optic wireless distribution network that enables seamless integration of the optical and wireless network infrastructures. Emerging wireless communication networks that support new broadband services provide increased opportunities for photonics technologies to play a prominent role in the realization of the next generation integrated optical/wireless networks. In this paper, we present a review of recent developments in radio-over-fiber technologies that can support the distribution of broadband wireless signals in a converged optical/wireless network. We also describe some of the challenges for the successful application of radio-over-fiber technologies in future wireless systems, such as 5G and 60-GHz networks.

Experimental demonstration of digitized RF transport over optical fiber links

We experimentally demonstrate high performance transmission of microwave signals deploying digitized RF transport over a standard optical fiber link. Bandpass sampling technique is used to digitize a 2.475 GHz microwave signal (WiMAX) with a 16 QAM modulation format and 6 MSymbol/s data rate and is transmitted over a 20 km optical link. Using an ADC with 8 bit resolution, a WiMAX signal was successfully transmitted and recovered with very good error-vector magnitudes (~1.77).

Efficient Transmission Scheme for AWG-Based DWDM Millimeter-Wave Fiber-Radio Systems

We propose and demonstrate an upstream transmission scheme using a semiconductor optical amplifier (SOA) for arrayed waveguide grating (AWG)-based dense wavelength-division-multiplexed (DWDM) millimeter-wave fiber-radio systems and show improved link performance. In our scheme, unused optical carriers from the cyclic AWG in the downlink (DL) are tapped for uplink (UL) transmission. An SOA in conjunction with the AWG simultaneously amplifies the DL RF subcarriers and UL optical carrier, thus improving carrier-to-sideband ratio in the DL while also yielding an improved power budget for the UL. Our experimental results show that the proposed scheme can be a practical solution for future bidirectional wavelength interleaved DWDM transmission systems

Optical Tandem Single-Sideband-Based WDM Interface for Millimeter-Wave Fiber-Radio Multisector Antenna Base Station

We propose and demonstrate a spectrally efficient wavelength division multiplexed (WDM) optical interface for millimeter-wave radio access networks with the capability of being integrated within a standard 100-GHz WDM infrastructure. The proposed WDM optical interface is realized by the use of a multiport optical circulator in conjunction with fiber Bragg grating filters. The interface supports demultiplexing of wavelength-interleaved optical tandem single-sideband modulated signals for a millimeter-wave fiber radio backbone with a sectorized antenna interface at the remote base station. The functionality of the interface is verified experimentally for RF signals at 29 and 31.6 GHz with 155-Mb/s binary phase-shift keying data. The effects of optical impairments on the transmission performance of WDM channels are experimentally analyzed. The experimental results demonstrate that the WDM optical interface performs efficiently with very low channel impairments due to optical crosstalk.

Performance Comparison of Directly Modulated VCSEL and DFB Lasers in Wired-Wireless Networks

We compare the RF power conversion efficiency using our analytical model for optical links composed of distributed feedback (DFB) and vertical-cavity surface-emitting laser (VCSEL) optical sources. We then experimentally compare RF power consumption and present link transmission performance for a directly modulated DFB laser-based and VCSEL-based hybrid access network overlaid on an ethernet passive optical network infrastructure for the distribution of wired and wireless services.

Experimental Demonstration of the Transport of Digitized Multiple Wireless Systems Over Fiber

We experimentally demonstrate high-performance transmission of multiple distinct microwave signals via digitized radio-frequency (RF) transport over a 20-km standard optical fiber link. A bandpass sampling technique is used to digitize a 2.475-GHz wireless signal [worldwide interoperability for microwave access (WiMAX)] carrying 6-MSymbol/s data in 16 quadrature amplitude modulation format and a 1.95-GHz wireless signal [global system for mobile communication (GSM)] with 270.833-kb/s data in Gaussian minimum shift keying modulation format. Using an analog-to-digital converter with 8-bit resolution, the WiMAX and GSM RF signals are simultaneously transmitted and recovered successfully with very good error-vector magnitudes of ~3.44 and ~1.42, respectively.