With the increasing demand for high information transmission rates, the existing frequency bands can no longer meet people’s needs, so researchers are starting to put the vision in the higher frequency band. However, a high frequency signal is difficult to achieve long distance transmission because of the high medium loss. In order to solve this problem, ROF (Radio over Fiber) technology, which combines the advantages of optical fiber communication and wireless communication, has attracted wide attention. ROF will be an important technique of high speed broadband wireless communication in the future, and has a broad application prospect. Therefore, each module of the ROF system is analyzed and studied in this dissertation. The main works are as follows:
1) The gain, noise and nonlinearity of ROF systems are analytically studied. The results show that using lossless matching network and reducing output resistor of the driver and the resistance of the laser diode can reduce the match loss of the transmitter. For the receiver, using a lossless match network, increasing the input resistor of the preamplifier can decrease the matching loss. In addition, the RF loss increases with the square of the optical loss, that is, 1dB of optical loss deduces 2dB RF loss. Second, the noise current and noise figure theoretical expression of the ROF and the relationship of the matching of the transmitter, laser bias current, optical loss, the matching of receiver, the noise properties of the preamplifier and the system noise figure are obtained. The curve of system SNR (signal to noise ratio) vs laser bias current is experimentally studied. The results show that by using lossless match network and reducing the output resistance of the driver and the series resistance of the laser, matching noise of the transmitter can be decreased, because the NF (noise figure) increases with the laser bias current, system noise can be decreased with the laser bias current reduction. For the receiver, the matching noise of receiver can be decreased by using a lossless match network, increasing the input resistor of preamplifier, reducing the conductance of the photon diode, and designing a low-noise preamplifier.
2) The SDD (Symbolic Defined Device) in ADS (Advanced Designed Systems) software is used to form the physical model of the laser. The model of lasers is very important for the design of a RoF link. By using the rate equation of lasers, the large signal and small signal models are analyzed. The mathematical model of the laser is established by using the Volterra series, and the derivation of the related nonlinear formula is also carried out.
3) A novel CMOS analogue broadband pre-distortion circuit fabricated in a 0.18-μm CMOS technology is firstly presented. Although there have been many reports about the predistortion technology for ROF lasers, but in recent 30 years, the analog predistortion circuit designed by CMOS technology is very few. If the predistortion circuit is designed in CMOS technology, and in accordance with the traditional predistortion structure, the whole circuit structure will be very complex, and it also needs a large chip area and consumes a large power. It cannot be tolerated with the system integration and the cost. The pre-distortion circuit proposed has the advantages of a simple structure, a low power consumption, and a small chip area. The problem that the predistortion circuit can not operate in broadband is solved. It realizes the breakthrough of CMOS analog predistortion technology for ROF systems. The measured results suggest that IMD3 has 3~16 dBc reduction over 1~2.5 GHz.
4) DNW-strip-SMPD and Psub-strip-SMPD are proposed. With the research of the photodetector’s working principle and performance index, a mathematical model analysis for the photodetector of the SMPD structure is established. Matlab is used to simulate the relationship between the 3-dB bandwidth and the PN junction distance. Two kinds of structure which can effectively improve the bandwidth of the photodetector are simulated by SILVACO software, which provides a basis for the design of CMOS photodetectors. The test results show that the working frequency of the DNW-strip-SMPD is 2.9 GHz while the Psub-strip-SMPD’s is 1.8 GHz. In addition, the parasitic capacitances of the DNW-strip-SMPD and Psub-strip-SMPD are around 820 fF and 440 fF, respectively. Psub-strip-SMPD has an advantage over the responsivity and parasitic capacitance, while DNW-strip-SMPD has better performance in 3-dB bandwidth.
5) A broadband programmable gain amplifier (PGA) for ROF links is proposed. The gain is variable by digital controlling the effective width and length ratio of the transistor. The test result shows that the 3-dB bandwidth of the broadband PGA is 0.9~5 GHz. The gain step is nearly 3 dB. At the highest gain, the minimum NF is 4.7 dB, while the minimum NF is 8.5 dB in the low gain mode. The IIP3 in the maximum gain mode is -9.6 dBm.