Adaptive control of feed-forward linearization for laser nonlinearity compensation system

Radio over Fiber technology (RoF) is a promising solution to the next generation wireless access network because of its ability to transmit high capacity data and to be cost effective. However, RoF systems are analog systems which are sensitive to noise and distortions. The RoF links need to have good linearity in order to avoid nonlinear distortions. The primary limitation on the performance of the optical transceiver in RoF links is the nonlinearity of the laser source in the transmitter. The laser source nonlinearities generate intermodulation distortion products which can severely degrade the performance of the RoF links. Hence, various linearization schemes are proposed to compensate the nonlinearity of the laser source, such as feedback, predistortion, and feed-forward. Among the linearization technique, feed-forward linearization is considered as the most effective due to its ability to provide broadband distortion reduction at high frequencies, and reduction in all order of distortions regardless of the laser nonlinear characteristics. However, feed-forward linearization is a relatively sensitive scheme, where its performance is highly influenced by changing operating conditions. Hence, the feedforward linearization system needs to be incorporated with adaptive properties in order to achieve optimization in linearization for more practical implementations. In this thesis, a laser transmitter feed-forward linearization system has been modeled in the commercial software OptiSystem 9.0. The laser transmitter feed-forward linearization system is integrated with the proposed adaptive control system developed in MATLAB through Visual Basic scripting. The results of the cosimulations have achieved significant reductions of over 20 dBm in the third-order intermodulation distortion products for operating frequencies from 5.1 to 5.8 GHz

Adaptive optical feedforward linearization of optical transceiver for radio over fiber communication link

With the tremendous growth in numbers of mobile data subscribers and explosive demand for mobile data, the current wireless access network need to be augmented in order to keep up with the data speed promised by the future generation mobile network standards. Radio over fiber technology (RoF) is a cost effective solution because of its ability to support numerous numbers of simple structured base stations by consolidating the signal processing functions at the central station. RoF systems are analog systems where noise figure and spurious free dynamic range (SFDR) are important parameters in an RoF link. The nonlinearity of a laser transmitter is a major limiting factor to the performance of an RoF link, as it generates spurious spectral components, leading to intermodulation distortions (IMD), which limit the achievable SFDR of the analog RF wave transmissions. The device nonlinearity can be mitigated through various linearization schemes. The feedforward linearization technique offers a number of advantages compared to other techniques, as it offers good suppression of distortion products over a large bandwidth and supports high operating frequencies. On the other hand, feedforward linearization is a relatively sensitive scheme, where its performance is highly influenced by changing operating conditions such as laser aging, temperature effect, and input signal variations. Therefore, for practical implementations the feedforward system has to be real-time adaptive. This thesis aims to develop an adaptive optical feedforward linearization system for radio over fiber links. Mathematical analyses and computer simulations are performed to determine the most efficient algorithm for the adaptive controller for laser transmitter feedforward linearization system. Experimental setup and practical measurement are performed for an adaptive feedforward linearized laser transmitter and its performance is optimized. The adaptive optical feedforward linearization system has been modeled and simulated in MATLAB Simulink. The performances of two adaptive algorithms, which are related to the gradient signal method, such as least mean square (LMS) and recursive least square (RLS) have been compared. The LMS algorithm has been selected because of its robustness and simplicity. Finally, the adaptive optical feedforward linearization system has been set up with digital signal processor (DSP) as the control device, and practical measurement has been performed. The system has achieved a suppression of 14 dB in the third order IMD products over a bandwidth of 30 MHz, in a two-tone measurement at 1.7 GHz

An improved laser transmitter using feedforward linearization technique for radio over fiber communication system

Several linearization techniques such as predistortion, feedback, quasi feedforward and feedforward which have been proposed are suggested to improve the linearity of a laser transmitter specifically for Radio over Fibre communication system. In general, feedforward linearization technique is known to be more effective since it can achieve reduction in all orders of distortion for large bandwidth and high frequency without needing to know the nonlinear characteristics of lasers. In this paper, we propose an improved feedforward laser transmitter design. The theoretical study shows that the proposed design is less sensitive since the distortion suppression is influenced by fewer parameters compared to other reported works. The simulation results show that the proposed system can achieve the optimum distortion reduction around 30 dB by only adjusting the coupling coefficient of both optical couplers in the distortion cancellation loop, while the phase is matched and the signal is cancelled optimally in signal cancellation loop. Previous works based on feedforward linearization technique will also be reviewed through some papers and patents

Adaptive control for laser transmitter feedforward linearization system

In this paper, an adaptive control system is employed in a novel implementation technique of the feed forward linearization system for optical analog communication systems' laser transmitter. The adaptive control system applies the Newton trust-region dogleg algorithm, which is a numerical optimization algorithm, to automatically tune the adjustment parameters in the feed forward loops to optimize the feed forward system performance and adapt to process variations. At the end of this paper, significant reductions of over 20 dBm in the third-order inter modulation distortion products have been achieved for operating frequencies from 5.0 to 5.8 GHz

Mathematical model of laser transmitter in feed-forward mitigating technique for radio over fiber system

This paper discusses a compensation system for nonlinear distortion of laser transmitter in Radio over Fiber application at frequency 5.2GHz, where feed-forward linearization technique is the applied technique in the compensation system. The paper focuses on the mathematical model of the laser nonlinear distortion which is an important tool in developing the feed-forward laser transmitter system model. The nonlinearity of the laser diode is modeled using Volterra series analysis, and the magnitudes of the laser 3rd order intermodulation distortion products (IMD3) are computed in MATLAB. The analysis result from the mathematical model has shown considerably good agreement with the results from some of the previous works. Hence, the mathematical model is deemed as useful in the modeling of feed-forward transmitter system

Intermodulation distortion analysis of feedforward linearised laser transmitter employing volterra series approach

This paper demonstrates intermodulation distortion (IMD) analysis of a feedforward linearisation technique in directly modulated semiconductor laser employing a Volterra series approach. Variations of IMD suppression with RF modulation frequency as well as modulation index are shown. The results show that the proposed system can reduce the third intermodulation distortion (IMD3) by more than 45 dB for the wide frequency range from 500 MHz to 10 GHz, 30 mA bias current and 0.1 modulation index. It is also shown that the IMD3 reductions remain constantly for variation of the index modulation from 0.1 to 0.9. Therefore the proposed system can reduce significantly IMD without influence of input power level