We present an empirically based model for the gain of an indoor antenna array. This corresponds to the re-parametrization of the Greenstein-Erceg model, as applied to 5 m-50 m narrowband outdoor-indoor links. Our model is applicable to a few adjacent tones in an OFDM femtocell system such as LTE. We find that as much as 14 out of 16 dB of antenna-gain are attainable even in rich indoor scattering conditions. At the same time, no larger fade margin is required when using the array vs. the omnidirectional antenna. To provide a reference for the observed results we consider a simple propagation model, which is analyzed theoretically and via simulation. This model is found to match our empirical results very well.