The Optimum Link Design Using a Linear PIN-PD for WiMAX RoF Communication

Worldwide Interoperability for Microwave Access (WiMAX) is a new standard for highspeed wireless communication that covers wider area than that of Wireless Local Area Network (WLAN). In the WiMAX, the original data are first mapped on the symbols of Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (QAM), or 64 QAM, depending on the data speed. The complex numbers of the symbols are allocated to subcarriers and the subcarriers are converted to time-domain I and Q data by Inverse Fast Fourier Transform (IFFT) algorithm. The parallel data of I and Q are converted to serial data by using a parallel to serial converter. The digital serial data are converted to analog data by using digital to analog converter. The analog I and Q data are orthogonally modulated by a carrier frequency and multiplexed to generate a time domain waveform. Therefore, in the Orthogonal Frequency Division Multiplexing (OFDM), the Peak-to-Average Power Ratio (PAPR) of the waveform becomes higher when the number of sub-carriers increases. When the waveform is converted by Electrical-Optical converter (E/O) to optical signal and transmitted over a fiber and is converted by Optical-Electrical converter (O/E) to electrical signal, the larger PAPR causes larger distortion in those optical components. Therefore it is strictly important to design the WiMAX communication link by using highly linear optical signal converters. Since the linearity in the actual optical components is insufficient to cover all modulation conditions in the WiMAX communications, the optimum design of the E/O, the O/E, the modulation, and the demodulation is necessity, based on the specific condition of the communication systems. But it is obvious to use the linear PIN photodiode (PIN-PD) for all of the WiMAX Radio-over-Fiber (RoF) links. Because, the structure and the performance are stable and simple, in comparison with that of other active optical components, such as Avalanche Photo diode (APD) and Distribute feedback Laser diode (DFB-LD).