I. Introduction
Due to the increasing penetration of smartphones and tablet-type devices, the demand for indoor wireless mobile communications continues to increase. More than 80% of mobile traffic in existing mobile cellular networks originates from or terminate inside buildings [1]. However, penetration losses and complex indoor environments make delivering high data rate wireless transmissions inside buildings a tough challenge, particularly in high-rise residential and office buildings, airports, shopping malls, and other indoor environments where concentrations of mobile users can be extremely high. Moreover, penetration losses are more prominent at higher frequencies. As new multimedia services and high data rate application intensifies, providing high quality indoor voice and data services becomes increasingly important.