Study On Key Technology In The Radio-over-fiber System

With the continuous development of the growing demand for communications, high-capacity and high speed are the basic requirements of the communication system for the future. Wireless access will be the mainstream of access technology because of the flexibility and convenience. The optical fiber communication system has wide bandwidth and small transmission loss, so it can provide all kinds of high quality services and can be also combined with the wireless access. It is the radio over fiber (RoF) technology.In the wavelength division multiplexed (WDM) systems, 1-bit delay interference approach does not work well for the DPSK signal demodulation, due to the approach has a high accuracy requirement of devices and is not transparent to the baseband data. In this paper, a phase modulated WDM-RoF link employing delay interference demodulation is proposed for the DPSK wireless signal transmission. The proposed link realizes error-free transmission, with the advantages of low precision requirement, structure simplification, and high flexibility.In order to avoid the usage of additional conversion devices for demodulation in the phase modulated RoF link, the fiber demodulation technology is proposed and investigated in this paper. The fiber demodulation technology can reduce the system cost and increase the system stability. Based on the fiber demodulation model, the bit error rate (BER) of RoF links with different modulation schemes is analyzed. The theoretical derivation proves that the DPSK can provide 3-dB high receiver sensitivity than ON-OFF keying (OOK) in RoF link.As the current modulation techniques can not solve the problem of transmitting vector signal in the RoF link, the optical multisideband modulation (OMSB) technology is proposed theoretically and demonstrated experimentally. The dispersion induced power fading effect can be almost overcome in the simple OMSB RoF link. The OMSB is the promotion of common optical double sideband modulation. In case of 50-Msymbols/s 32-quadrature amplitude modulation signal with subcarrier frequency at 40 GHz over 100-km single mode fiber, the RoF-link's error vector magnitude of better than 4.5% is obtained experimentally.Additionally, a power balanced element using in the true time delay network is presented and implemented. The 7-dB optical power jitter can be steadied into 0.2dB by the power balanced element, which markedly improves the network quality.