Research On The Key Technique Of Radio Over Fiber Transmission

As Radio over Fiber (RoF) has the characteristics of removable, large capacity of datatransmission and abundant bandwidth resources, it has become the research focus ofcommunications industry. However, the distortion of signal exists due to the nonlinearity in theRoF system. Especially for the non-constant envelope modulation signal with large peak-to-average-ratio, it will result in spectral re-growth, adjacent channel interference and deterioration ofthe performance in a RoF system. On the other hand, due to the advantages of low cost and easy toimplement, the digital pre-distortion technology is widely used in a radio frequency poweramplifier system. This thesis attempts to resolve the problem about the nonlinearity of the RoFsystem through digital pre-distortion. The thesis is not only introduces the look-up-table pre-distorter for linearing the RoF system, but also gives the Augmented Hammerstein pre-distorterwhich has the ability of dynamic nonlinear compensation to linearize the RoF system.This work can be divided into the following three parts:In the first part, development of linearity techniques, which are applied to the RoF system isintroduced. The principle of the nonlinear distortion and several digital pre-distortion modules havebeen elaborated here.The second part gives digital predistortion linearization simulation and experiment in a RoFsystem. Firstly, the RoF system is set up, which is based on the laser and10Gbps photo-detector.After that, gain and phase characters have been tested by vector network analyzer, the test resultsshow that the1dB compression point lies at-3.2dBm. Then, the feedback signal of a RoF system isextracted and the delay time is estimated, and the extracted parameters are imported to build alookup table and dynamic linear subsystem. After that, the AM/AM and AM/PM characteristicsare analyzed. Finally, the pre-distortion signal is generated in ADS.The experimental results are discussed in the third part. Two experiments have been donewith the three-carrier WCDMA (Wideband Code Division Multiple Access) signal and the singlecarrier LTE (Long Term Evolution) signal. The results of two experiments are given below. Whenthe WCDMA signal is used in the experiment, the ACPR (Adjacent Channel Power Ratio) of theRoF system will be improved7.2dB and9.2dB after linearizing by look-up-table and theAugmented Hammerstein pre-distorters, respectively. For the single-carrier LTE signal, the ACPRimproved9dB and11.4dB with obvious linearization effects. Since there is dynamic nonlinearity in a RoF system, Augmented Hammerstein pre-distorter has better ability to compensate thenonlinearity.