Downlink performance of two-tier networks with time-hopping modulation

With growing demand for high data rates, the mobile network has faced excessive traffic and it seems that one of the few long-term solutions to this problem is employing femtocells which improve capacity and indoors coverage. In the typical femtocell networks, co- and cross-layer interferences caused by simultaneous transmission of macrocell base stations and femtocell access points in the same frequency band, results in dead spots especially if femtocells are densely employed. In this study, to mitigate the interference and improve the system´s performance, the authors study the application of time-hopping approach. Particularly, the impact of aggressive signals of femto access points on a macro user equipment located in the second tier, that is, the femtocell coverage area, is analysed and the semi-closed-form expression for the system outage probability is derived. Based on the analysis, they also simulate the transmission capacity (TC) that represents the total capacity of the co-channel two-tier networks with outage constraints. Next, employing Gaussian approximations, they derive upper and lower bounds on the outage probability. These bounds provide better insight on the effect of system´s parameters on its performance. Their numerical results show that the time-hopping technique significantly reduces the outage probability and therefore increases the TC of two-tier networks.