The authors suggest a mathematical formulation to describe the random mobility of users in microcellular systems. Variable user mobility is characterized with an analytical model where users are assumed to change their moving speed and direction randomly. The study on user mobility enables us to improve the accuracy of performance analyses for channel utilization. Utilizing the proposed approach for user mobility, we provide a more realistic mobility model and study the probability distributions of cell sojourn time (CST) and channel holding time (CHT) considering random user mobility.