Ampalavanapillai Nirmalathas

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.

Analysis of optical carrier-to-sideband ratio for improving transmission performance in fiber-radio links

In this paper, we investigate the optimum carrier-to-sideband ratio (CSR) for maximizing the transmission performance of an optically modulated millimeter-wave signal in a fiber-wireless system via experiment, theory, and simulation. We present a simple analytical model to assess the performance enhancement resulting from optical CSR variations. The model is capable of analyzing systems incorporating binary phase-shift keyed and quaternary phase-shift keyed modulation formats. We quantify the optical CSR of a point-to-point fiber-radio link and establish that the performance of the fiber-wireless links can be significantly improved when the optical signal is transmitted at the optimum CSR of 0 dB. The analysis further shows that the optimum optical CSR is independent of transmission bit rates.

Transmission improvement in fiber wireless links using fiber Bragg gratings

We demonstrate a simple, passive technique for significant improvement of transmission performance in fiber wireless links through the application of a narrow-band fiber Bragg grating. The grating is used to optimize the optical modulation depth in the transmitted signal by reducing the power of the optical carrier. We present experimental measurements that show improvements in receiver sensitivity (at bit-error rate =10/sup -9/) of up to 7 dB for both double- and single-sideband modulation schemes. The scheme is applicable to a wide range of radio frequencies and modulation depths.

Intermodulation Distortion Improvement for Fiber–Radio Applications Incorporating OSSB+C Modulation in an Optical Integrated-Access Environment

In this paper, we investigate the reduction of intermodulation distortion (IMD) in fiber-radio systems incorporating a dispersion-tolerant optical single sideband with carrier modulation. We present a systematic analysis and quantification of the third-order IMD generated due to optical components in the nonlinear optical front-end. Our proposed technique to improve the optical front-end linearity is by the removal of the optical components that contribute most to the third-order IMD in the RF domain. We experimentally demonstrated the proposed technique with two- and three-tone tests and showed more than 9-dB improvement in the overall carrier-to-IMD ratio. The proposed technique was also investigated via simulation analysis for a larger number of radio channels and showed an IMD suppression of >10 dB. In addition, the proposed technique is not only able to improve the carrier-to-interference of the radio signals but also to enable simultaneous baseband transmission, thereby facilitating the merging of millimeter-wave fiber-radio systems with other wired-access infrastructure. We present a detailed investigation and characterization of this technique.

Millimeter-wave broad-band fiber-wireless system incorporating baseband data transmission over fiber and remote LO delivery

We present the first demonstration of a millimeter-wave (mm-wave) broadband fiber-wireless system which incorporates baseband data transmission in both the downstream (622 Mb/s) and upstream (155 Mb/s) directions. The local oscillator (LO) required at the remote antenna base station for up- and downconversion to/from the mm-wave radio frequency (RF) is delivered remotely via a modulation scheme that is tolerant to the effects of fiber chromatic dispersion on the detected LO carrier power. The technique employs a single dual electrode modulator located at the central office (CO) and the data and an RF signal at a frequency equal to half the LO frequency, are applied simultaneously to the device. The modulation scheme was optimized as a function of the modulator operating conditions. Simultaneous bidirectional radio transmission in the mm-wave fiber-wireless network was implemented using specially designed mm-wave diplexers located at the base station (BS) and customer unit, and a single Ka-band printed antenna array at the BS operating simultaneously in transmit and receive mode. Error-free data transmission was demonstrated for both down(34.8 GHz) and uplinks (37.5 GHz) after 20 km of single-mode optical fiber and a bit error rate (BER) of 10/sup -6/ was achieved after the inclusion of a 2-m radio link.

High-Speed Optical Wireless Communication System for Indoor Applications

A novel high-speed optical wireless communication system for indoor personal area networking applications is proposed and studied. A proof-of-concept experiment at 12.5-Gb/s wireless transmission has been successfully demonstrated with limited mobility. When integrated with a WiFi-based localization system, high-speed optical wireless communication with a mobility feature can be achieved over the entire room. The performance trade-offs between the maximum beam footprint and overall bit rate have also been studied and quantified experimentally and the results show that error-free (BER<;10-9) reception can always be achieved for a wide range of bit rates from 1 to 12.5 Gb/s.

Efficient multiplexing scheme for wavelength-interleaved DWDM millimeter-wave fiber-radio systems

A simple multiplexing scheme is proposed and demonstrated with the capability to interleave optically modulated 37.5-GHz radio channels in a dense-wavelength-division-multiplexed fiber-radio system with 25-GHz wavelength spacing, and also enable a carrier subtraction technique that improves the overall link performance by reducing the carrier-to-sideband ratio of the multiplexed channels. The proposed scheme is realized by the use of an arrayed waveguide grating having multiple optical loop-backs between the input and the output ports.

Wavelength reuse in the WDM optical interface of a millimeter-wave fiber-wireless antenna base station

A novel technique for wavelength reuse has been proposed to simplify the upstream optical interface of an antenna base station in a millimeter-wave fiber-wireless system incorporating wavelength division multiplexing. This technique is based on recovering the optical carrier used in downstream signal transmission and reusing the same wavelength for upstream signal transmission. Two novel configurations for optical carrier recovery and wavelength reuse are proposed and demonstrated experimentally.

Performance analysis of optimized millimeter-wave fiber radio links

We present a comprehensive performance analysis of several optimized fiber radio distribution schemes for millimeter-wave radio services. The analysis includes the noise and nonlinear characteristics of the transmitter (Tx)-receiver (Rx) pair integrated with the analog optical link in the downlink transmission of a given wavelength without optical amplification. Investigations are focused on four configurations of optimized fiber radio links, which were derived by considering the best performing possible Tx-Rx configuration and specifications of commercially available devices to support multichannel subcarrier multiplexed transmission. It was found that the nonlinear characteristics of the Mach-Zehnder modulator are the major source of performance degradation of the fiber radio links. A comparison of RF-over-fiber and IF-over-fiber transport schemes also shows that RF-over-fiber can yield 3-dB improvement in performance compared to IF-over-fiber techniques.

Tunable All-Optical Wavelength Conversion of 160-Gb/s RZ Optical Signals by Cascaded SFG-DFG Generation in PPLN Waveguide

We report, for the first time, tunable all-optical wavelength conversion of 160-Gb/s return-to-zero (RZ) optical signals based on cascaded sum- and difference-frequency generation in a periodically poled LiNbO3 waveguide. The distorted signals due to limited phase-matching bandwidth during conversion were compensated by spectral reshaping. We achieved error-free tunable wavelength conversion with a bit-error rate of less than 10-9 for 160-Gb/s RZ signals in a 23-nm tuning range over the C-band