Indoor positioning using RF signals is usually a difficult task, due to multipath phenomena, that, especially for narrowband transmissions, can deeply affect signal propagation. In this paper, a simple technique for improving the accuracy of a received signal strength based ranging system is proposed. It is shown that, using narrowband low power transmissions in the 2.4 GHz channel and a multiple input multiple output approach, multipath effects can be mitigated, leading to a significant improvement in accuracy.