Beamforming technique is used in IEEE 802.15.3c networks to avoid high propagation attenuation and path loss, and improve the overall system throughput by exploiting spatial channel reuse. In this paper, we introduce the necessity and design challenges of inter-piconet communications. Based on our observation, we propose a joint design of relay selection and scheduling. For relay selection, we define "interference-free", "concurrence-restricted extent", and "concurrence index" for properly selecting relays to improve the degree of spatial channel reuse. The designed scheduling consists of two phases: intra-piconet flow scheduling and inter-piconet flow scheduling. After performing phase 1 operations, intra-piconet flows are scheduled and the total required channel time is known. The phase 2 operations are to use the least channel time extension to schedule all inter-piconet flows. We evaluate our algorithm through simulations. The results show that the joint algorithm performs well and does achieve its objectives.