Printer identification based on a printed document can provide forensic information to protect copyright and verify authenticity. In this work, a stochastic dot interaction model that take in to consideration of scanner characteristics and print-scan channel noise is developed to predict the impact of embedding extrinsic signatures using laser intensity modulation. With this model, reflectance of the printout can be effectively estimated without extensive measurements. In addition, we proposed an optimization framework to select the modulation parameters that will maximize the embedding capacity and detection reliability. Preliminary analysis results such as achievable capacity and correct detection rate are discussed.