Contents 1. Introduction
In the digital wireless communication of Worldwide Interoperability for Microwave Access (WiMAX) [1], Long Time Evolution (LTE), Wireless Fidelity (WiFi), etc. optical fiber transmission is used between central office and base stations. In the conventional optical fiber transmission, Orthogonal Frequency Division Multiplexing (OFDM) signal is converted to digital signal by Analog Digital (A/D) converter and transmitted through the optical fiber. In this transmission, by introducing Radio-over-Fiber (RoF) technology which transmits un-digitalized wireless-signals of OFDM, Remote-Antenna-Unit (RAU) of small and simplified base station is realized. This RAU configuration transfers the modulator, oscillator, the A/D converter, and the control system to the central office from the base stations [2]. Consequently, the RAU mainly consists of Electrical / Optical (E/O) converter, Optical / Electrical (O/E) converter and electrical power amplifier. By using WDM (Wavelength Division Multiplexing) technology for the RoF link between the central office and base stations, heterogeneous signals of WiMAX, LTE, and WiFi can be transmitted through single optical fiber, therefore various applications will be expected [3]–[5]. To realize this technology, it is important to investigate how the wavelength spacing is defined and how many channels are possible for the WDM. Wideband modulation and long distance transmission can be achieved by using 5GHz modulation and 1550nm optical wavelength band for the WDM. Since the cost of high frequency modulation E/O transmitter using Electro-Absorption-Modulator (EAM) is high, a design of cost reduction has to be carried out for the WDM E/O transmitter. When reducing the wavelength spacing to increase the channels number of WDM for the RoF, optical interference between two different signals of WiFi and WiMAX may occur. Therefore, we have realized a lower cost 5GHz-LD for WDM by modifying conventional low-cost 2.5Gb/s-digital-coaxial Laser Diode (LD), and designed a specification of optimal wavelength spacing and frequency spacing, by evaluating error in the WDM and Frequency–Division-Multiplexing (FDM) modulations of WiMAX (IEEE802.16) and WiFi (IEEE802.11) signals.
