Comparison between Direct and External Modulation Methods in the RoF Analog System

• Main Author: Walaa AL_Smadi
• Other Authors: San-Liang Lee
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/72175154081933902779

碩士 === 國立臺灣科技大學 === 電子工程系 === 100 === The next generation access networks are meeting wire line and wireless services. To offer the end users more choices and greater suitability, it is desirable to provide high speed connection to serve the fixed and mobile costumers with high capacity, large bandwidth, and good signal, for the radio over fiber system. Actually, two types of modulation techniques are used in analog radio over fiber system. The direct modulation technique, which is easy to demonstrate and has a low cost. But, it cannot provide a high gain because of the laser limitation. On the other hand, external modulation laser, which operates in continuous wave mode. This method can provide a high gain. But, it is complex and expensive. This thesis makes a comparison between the direct and external modulation for radio over fiber application. The advantages and disadvantages for both types of modulation techniques are discussed with respect to the following parameters: gain, maximum radio frequency input power, and the power budget. It also introduces some definitions and assumptions in brief. Furthermore, it proposes some techniques which can improve the link performance. From the link performance analysis we found that the good EVM value for direct modulation is obtain in the RF input power range of -1.4 dBm to 13.6 dBm, unlike external modulation which ranges between to -7.4 and 4.6 dBm. In the case of external modulation, the increasing in optical input will decrease the RF input power and at the same time will increase the RF output band power range; as a result, we conclude that direct modulation is suitable for the downstream architecture which needs a higher RF input power to transfer the data. While, external modulation can be used for the upstream architecture, since it needs a high optical input power to transfer the data from the users to the base station. We found that under the maximum conversion efficiency condition the RF loss is far better for direct modulation than external modulation, in terms of PD optical input power and the RF input power. Measuring the power budget for the RoF links with a splitter of 2, 4, 8, 16, and 32 splitting ratio, the external modulation can achieve a 4 dBm gain at 100 mW optical power with using EDFA in the system, unlike the direct modulation which can obtain -11 dBm loss under the same condition. Measuring the spurious – free dynamic range for RoF link shows that the external modulation of 119 dB. Hz^(3⁄2) has a better linearity than the direct modulation (79.81dB. Hz^(3⁄2)).