In this paper the performance of multiple-input multiple-output (MIMO) system in the UTRAN Long Term Evolution (LTE) standard is evaluated by means of ergodic system capacity in low (3 km/h), medium (30 km/h) and high (120 km/h) mobility scenarios. The impact of adaptive modulation as well as Hybrid ARQ (HARQ) with Chase Combining to the distribution of effective SINR is established analytically and validated by simulations. A model for ergodic system capacity employing adaptive modulation and HARQ is developed. The simulation results show that adaptive modulation combined with HARQ increases the MIMO system performance significantly especially in low mobility case, and smaller reduction of rate is experienced in high mobility scenarios.