Relay-Aided Outdoor-to-Indoor Communication in Millimeter-Wave Cellular Networks

In this article, we examine the feasibility of serving an indoor mobile terminal (MT) by an outdoor base station (BS) in urban millimeter-wave (mmWave) cellular networks. Toward this, we consider two scenarios: 1) the signal reaches the reference MT directly by penetrating the building where it is located and 2) the communication is assisted by a relay placed on the rooftop of the building that conveys through a waveguide the desired signal to an indoor transmit antenna located at the floor where the reference MT is located. By using a stochastic-geometry-based framework, we derive for both scenarios, analytical expressions for the coverage and average rate, which exhibit a good match with respect to the simulations. In addition, from these expressions, we gain important insights regarding the performance comparison of the two scenarios. In particular, it is analytically proved and validated by the simulations that relaying notably enhances the performance of the low-rate MTs even under substantial waveguide losses. However, for such losses, the average rate in the relaying case can become worse than the case without relays.