Design and FPGA Implementation of an Adaptive Digital
Predistortion for Radio-over-Fiber links
Even though the application of radio-over-fiber (ROF) links is widely used in wireless communication, nonlinear distortion (NLD) limits their dynamic range. In this paper, hardware implementation of adaptive digital predistortion system for ROF links is described. This complex gain predistortion technique is used to compensate amplitude and phase distortion. The system is adapted from the difference between the input and the output base band signals. This system supports to keep tracking of modifications and drifts in the link characteristics. Implementation using Xilinx's Vertex2 FPGA and test with SMIQ, WCA, logic analyzer are accomplished.
Over the past decade there have been huge advances in the areas of mobile and optical fiber communications. Specially, by using Radio-over-Fiber (ROF) technology, the ability of optical networks can be combined with the flexibility and mobility of wireless access networks more easily and at lower cost. The Radio-over-Fiber concept means to transport information over optical fiber by modulating the light with the radio signal either at the carrier frequency or at an intermediate frequency. This technique combines two media: radio and optical. The optical part is used to interconnect a central radio processing facility with a remote radio antenna and the radio part gives coverage to wireless broadband users. The ROF system offers low signal attenuation in the fiber, improved coverage and system performance, enhanced capacity, low RF power loss, and reduced complexity due to the centralized processing of RF signals.
However, the performance of ROF systems can be severely affected when the wireless link part of the system is in series with the ROF link, especially in a multi-user condition. Nonlinear distortion of the ROF link, due to mainly to the laser diode becomes the biggest concern. Also, nonlinearity of RF amplifier can be another source for distortion. Furthermore, a result of path loss, fading, and shadowing in the wireless link affects the nonlinearity of ROF links. In order to have large linear region, these nonlinear distortion must be compensated. Several techniques have been proposed to solve the problem of the nonlinearity. One approach is finding equivalent laser model equation or circuit models and using the inverse model of it to the system. However, even the equation and parameters found are quite accurate, this equation and parameter are device dependent. Another approach proposes squeezing the signal within the linear range of the ROF link. This technique compensates amplitude distortion but phase distortion will still exist within this so called linear region. Also, it needs extra amplification at the receiver. Some authors have used adaptive signal processing algorithms to generate an inverse model to eliminate the nonlinearity when placed in cascade. In this paper, look-up table (LUT) based adaptive digital predistortion technique is described to solve nonlinearity of the ROF links. This technique predistorts the input signals to compensate the amplitude and phase distortion of the ROF link. The predistorter has a nonlinear transfer function with gain expansion that is the inverse of the ROF link gain compression, and a phase rotation that is negative of the ROF link phase rotation. Adaptive digital predistortion technique seems to be a promising approach because it is cost effective and relatively simple. Also, this technique is re-programmable, device-independent and can follow variations of device parameters. This project is involved to a FPGA and ASIC prototype system which uses digital signal processing (DSP) techniques to implement an adaptive digital predistortion scheme that corrects for the ROF link's nonlinearity.