Dynamic range improvement techniques for fiber-optic microcell radio systems

This paper presents radio techniques to improve the input dynamic range for fiber-optic microcell radio systems. Several kinds of countermeasures for intermodulation distortion are discussed. FM double modulation (FM-DM) is highlighted to show that it improves the input dynamic range. First, the theory of FM-DM is presented. Second, the carrier-to-noise ratio (CNR) and the 3rd order intermodulation distortion (IM3) are shown theoretically and experimentally compared with conventional direct intensity modulation. By using FM-DM, even a commercial Fabri-Perot LD experimentally achieves an input dynamic range of 60 dB, which is superior to the conventional method by 20 dB. Moreover, the use of FM-DM improves the sensitivity of the received signal power by 23 dB in the experiments using π/4-QPSK signal