Subcarrier optical transmission techniques for wireless local loop (WLL)

The WLL system has some advantages in that the access equipment is economical and the installation is easier and faster. A cell station (WCS) for the WLL is generally constructed of two components, one is made up of the radio transmitter and receiver, and the other is made up of the modulator/demodulator (MODEM) and other devices. Subcarrier optical transmission techniques can reduce the size of WCS and decrease power consumption, since the MODEM equipment is set up at a telephone center. However, laser diodes (LDs) utilized for an electric-to-optic (E/O) conversion are very expensive, because of the demand for linearity and low noise characteristics. In this paper, the application of signal extraction and frequency arrangement (SEFA) technique along with a new idea, i.e. a level compression technique among input signals (LCS), are discussed. As a result, it is clarified that an economical Fabry-Perot LD (FP-LD) can be applied to the subcarrier optical transmission techniques, even though their linearity and noise characteristics are worse than those of the distributed feedback LD (DFB-LD).