Abstract

ABSTRACT

Analog Pre-distortion Circuit for Simultaneous Suppression of Third and Fifth Order Intermodulation Distortion in Broadband Radio-over-Fiber Systems

Shuvasish Saha

Rapid advance in wireless technologies coupled with the exponential increase in use of high bandwidth devices and applications have made it necessary to develop high capacity wireless transmission networks. Design of robust and cost-effective wireless signal transmission systems has become of paramount importance to keep up with the breakneck pace of wireless access demand, especially keeping in mind the future of massive multiple-input and multiple-output (MIMO) wireless. Radio-over-Fiber (RoF) transmission systems are at the forefront of the research topics being investigated right now as a possible solution to keep up with this exploding demand for wireless network access.

Based on optical subcarrier modulation, RoF transmission systems combine the use of both optical fiber and radio transmission. Optical fibers are low cost. They are lightweight and suffer from low loss. They provide extremely high capacity and immunity from electromagnetic interference. Radio transmission over fiber makes it easy to assemble Remote Radio Units (RRUs) at antenna towers. However, RoF is an analog optical transmission system and it is susceptible to non-linear distortions caused by all the inline functional optical and electrical components. In case of front-haul RoF transmission systems of wireless access networks, two key specific functions: RF power amplification and optical subcarrier modulation, are the main contributors to the production of non-linear distortions. Non-linear distortions consist of harmonic distortions (HDs) and intermodulation distortions (IMDs). It is essential to suppress these distortions because they can introduce crosstalk if they fall in the passband of RF signals.

In this thesis, the use of analog pre-distortion circuit (PDC) is investigated as a linearization technique for the purpose of suppressing non-linear distortions, especially the third order intermodulation distortion (IMD3) and the fifth order intermodulation distortion (IMD5) simultaneously. An analog pre-distortion circuit (PDC) is designed and fabricated based on the transmission characteristics of a modulator integrated distributed feed-back (DFB) laser (EML). The PDC is low cost, compact and has broad operational bandwidth. It is designed to operate in the bandwidth up to 6 GHz, using two beam-lead silicone Schottky diodes as predistorter. The linearization by this PDC is verified in EML modulated RoF transmission system. First, by using the EML modulated RoF, Spurious Free Dynamic Range (SFDR) improvement of over 11 dB is achieved related to IMD3 and over 3 dB related to IMD5 for the entire bandwidth. Similarly, the SFDR improved by more than 6 dB related to IMD3 and by more than 4 dB related to IMD5 for another EAM modulated RoF throughout the entire bandwidth. When Wi-Fi signals at 2.4 GHz and 5 GHz were transmitted through the EML modulated RoF, error vector magnitude (EVM) was improved by 1.3 dB at 2.4 GHz and by 1.55 dB at 5 GHz for back-to-back (BTB) transmission. For 10 km single mode fiber (SMF) transmission, EVM improved by 1.07 dB at 2.4 GHz and by 1.03 dB at 5 GHz. For the EAM modulated RoF, EVM was improved by 2.76 dB at 2.4 GHz and by 1.45 dB at 5 GHz for BTB transmission. For 10 km SMF transmission, EVM was improved by 2.08 dB at 2.4 GHz and by 1.44 dB at 5 GHz. Moreover, Wi-Fi signals were generated at a RF carrier of 2-5 GHz and EVM improvements were recorded. For the EML modulated RoF, EVM improvement was more than 1.4 dB for BTB transmission and more than 1 dB for 10 km SMF transmission. For the EAM modulated RoF, EVM improvement was more than 1.4 dB for both BTB and 10 km SMF transmission.