Research On The Information Transmission Technology Of The Radio Over Fiber

To meet the bandwidth demand of the wireless services, e.g. video on demand, internet mobile access and the other new multimedia service, it is necessary to use the microwave/millimeter wave signals with higher frequency to increase the capacity of the wireless communication systems. The key techniques for the application of wireless communications with the high frequency include the reduction of cost of the base station, the centralization of management for the bass station and the transparent transmission of the signals with the different data rates and modulation formats. Radio over fiber (RoF) technique, which uses fiber-optic links to transmit and distribute radio signals, effectively combines the advantages of the high bandwidth, the low loss, immunity to the electromagnetic interference provided by the fiber optic communications and the high flexibility provided by the wireless communications. It is a powerful solution to the future broadband wireless communications.The generation of the radio over fiber signals and their transmission performance over the fiber are investigated. The main contributions of the dissertation are listed as follows:1. The performance of overcoming the dispersion for the different modulation schemes of the baseband data is systematically analyzed in the radio over fiber system. It is pointed out that for the optical single sideband with carrier modulation, the RoF systems with different baseband data modulation schemes have the entirely different performance of overcoming the dispersion. The scheme of OOK modulating the optical carrier of the baseband signal makes the baseband signal carried by the both of the optical components of the OSSB+C signals. Due to the dispersion, they lose syschronization when they reach the photodiode, resulting in the waveform distortion. The scheme of PSK modulating the RF subcarrier of the baseband signal makes the data carried only by the optical sideband, avoiding the waveform distortion resulting from the fiber dispersion. The simulation results show that when the baseband modulation schemes are different, the RoF systems also have the different performace of overcoming the fiber. In addition, for the ODSB modulation, when the base signals modulate the optical carrier or the RF subcarrier with PSK, the RoF could not work.2. For the large carrier to sideband ratio (CSR) of the optical single sideband with carrier (OSSB+C) signals (genrally, as high as 15～20 dB), an improved structure of MZM with three modulation arms is proposed. For the any small, medium, and large modulation index, the power of the optical carrier component of the output optical signals is controlled by adjusting the DC bias voltage of the DC arm in the MZM, and the optimum CSR of 0 dB is achieved. An improved method to generate the OSSB+C signals is also presented based on the MZM with four modulation arms. By adjusting the DC bias of the main MZM, the OSSB+C with the optimum CSR of 0 dB is generated for any modulation index. Compared with the published methods, the methods proposed in this paper are developed from the view of the modulation, and they are simpler and more flexible in structure, and better in scalability.3. The full-duplex ODSB RoF system to overcome dispersion based on the MZM with four arms is proposed. In the ODSB signals exported from the MZM, one optical sideband carries the basedband data, while the other one is pure (carrying no data). Therefore, the waveform distortion is avoided; in the meanwhile, part of the pure optical sideband or part of the optical component carrying the data with the special modulation format are both as the optical source of the uplink, then the cost of the base station is reduced, and the full-duplex RoF information transmission is achieved.4. Two optical generation methods of mm-wave signals with frequency multiplication are proposed. It is attractive to generate the mm-wave signal with frequency of x×fRF based on the low speed RF signal with frequency of fRF, which degrades the demand of electronic and photonic devices. A. novel generation method of mm-wave signal with frequency tripling based on the MZM with three arms is proposed, in which the desired mm-wave signal with frequency tripling is obtained by beating of -1 (-2) order optical sideband and +2 (+1) order sideband in the receiver. A method of generating mm-wave signal with frequency quadrupling is also proposed using the MZM with four arms based on a 1×4 MMI coupler, in which the mm-wave signal with frequency quadrupling is obtained by beating of -1 (-3) order optical sideband and +3 (+1) order sideband. Both of the proposed frequency multiplication methods are simple in structure, and good in scalability. In the meanwhile., since the MMI coupler has the prospects of the equal power and the inherent optical phase relation between the output ports, the number of the couplers and the DC bias in the modulator is largely reduced, which degrades the complexity of the MZM.