Abstract— A radio over fiber (RoF) communication system is considered for cable television (CATV) channels over wavelength division multiplexing (WDM) network using an optical direct modulator (DM). Since the DM bandwidth is limited, increasing bandwidth efficiency of the system using M-ary quadrature amplitude modulation (MQAM) schemes will improve the link symbol error rate (SER) performance. Radio frequency (RF) signals with constellations of 16-64-256 QAM were simulated for CATV channels and the effect of using higher bit per symbol on the link performance was studied in order to reduce the cost of the network through utilizing financially feasible optical modulator. The 64-QAM scheme which is the most appropriate is used for the system of CATV channels. The link performance is studied for different combinations of effective parameters such as AC electrical power and Module to Bias ratio, which are crucial to be set properly, and SER is obtained at the front end of the receiver. The best setting options are proposed for 30 CATV channels with 64-QAM constellation with direct modulation transmitted over the 4-WDM link. Moreover, communication system is modeled by fuzzy logic using data-base obtained from simulation. Fuzzy reverse modeling of the system is also implemented in order to provide reasonable estimate of electrical power required to achieve a desired SER. Characteristics of forward and reverse fuzzy modeling are discussed. Simulation results represent satisfactory performance of the models. Index Terms— Direct Modulator (DM), Fuzzy Logic, Fuzzy Reverse Modeling, Radio over Fiber (ROF) I. INTRODUCTION or accommodating with the increasing demand of transmitting multiple data channels simultaneously through a fiber link, efficient and economic network design is valuable. Different optical modulation methods can be used for transferring CATV signals from electrical domain to the optical [1]. Only some modulation methods are cost-effective Manuscript received July 22, 2012. Maryam Niknamfar is with the University of Texas at San Antonio, San Antonio, TX 78249 USA, (phone: 210-322-7803; e-mail: ldx557@my.utsa.edu). Yashar Sahraei Manjili, is with the University of Texas at San Antonio, San Antonio, TX 78249 USA, (e-mail: sca102@my.utsa.edu). Mohammad Jamshidi is with the Electrical Engineering Department, University of Texas at San Antonio, San Antonio, TX 78249 USA, (e-mail: moj@wacong.com). Mehdi Shadaram is with the Electrical Engineering Department, University of Texas at San Antonio, San Antonio, TX 78249 USA, (e-mail: Mehdi.Shadaram@utsa.edu). and reliable. We have considered various electrical M-QAM schemes for transmitting 30 CATV channels over a 4-WDM RoF network. The cost effective optical modulation method for a WDM network with four or less routes is utilization of DM for each route [2]. However, bandwidth of RF signals, laser clipping, chirping and nonlinearity effects can limit the quality of the link performance [3]. Using DM laser is cheaper than using the external modulator. But in return, a DM laser with a higher bandwidth will be needed to support the RF signal spectra. The higher the bandwidth of the DM laser, the higher the cost. On the other hand, increasing the number of bits per symbol will decrease the required laser bandwidth and will consequently reduce the cost; higher M-QAM brings more SER in the symbol detection stage. Hence, there should be a tradeoff between using laser with higher bandwidth and utilizing higher M-QAM scheme in order to achieve the optimum link SER while reducing cost of the network. Furthermore, different system adjustments in order to achieve a linear performance of the direct modulator laser are important for analog transmission of RF signals [4]. Some parameters such as electrical power of the RF signals and DM laser characteristic parameters may result in linear or non- linear behavior of the DM laser. Also, an appropriate laser biasing prevents undesirable effects of the laser clipping, nonlinearity, and affects the modulator bandwidth based on the rate equation modeling [4]. Fuzzy modeling uses the actual input output data of the system to imitate its behavior [5], while, Fuzzy reverse modeling deals with outputs of the system or a mixture of some inputs with the outputs of the system in order to estimate the required values of other remaining inputs. II. OVERCOMING THE DM LASER LIMITATIONS THROUGH OPTIMIZED SYSTEM DESIGN For increasing the efficiency of a system’s bandwidth, using higher number of bits per symbol can be a solution. Bandwidth efficiency is defined as follows [6]:   =    = 1   // ;  = log 2  (1) Where R is the bit rate, W is the bandwidth, T s , T b, K and M represent symbol duration, bit duration and number of bits per symbol and symbol alphabet size, respectively. Drawback of increasing K is the increase in bit error rate which in case of Cost Effective ROF Communication System for CATV Channels over WDM Network and Fuzzy Modeling of the System Maryam Niknamfar, Student Member, IEEE, Yashar Sahraei Manjili, Student Member, IEEE, Mohammad Jamshidi, Fellow, IEEE, Mehdi Shadaram, Senior Member, IEEE F