Millimeter wave (mmWave) wireless personal area networks (WPANs) could provide multi-Gbps based data rate for short range communications. However, this ultra high data rate is sensitive to the co-channel interference (CCI). In this paper we use synchronization frame (sync frame) to mitigate possible interference between WPANs. Sync frames could be simply considered as the copies of beacons, which are relayed via the devices associated with the piconet. By using sync frames, the possible CCI could be reduced by increasing per WPAN coverage range, but meanwhile, the spatial reuse capability is also reduced. Therefore, in this work, we are motivated to investigate the optimized beacon/sync frame range at different link distances and its effect to throughput. Simulation results in 60 GHz system for a 2.5 m link indicate that, sync frames can double the guaranteed throughput. For guaranteed throughput of 1.4 Mbps at a 3.5 m link, beacon range can be 35% reduced. Another contribution of the paper is investigating the effect of CCI in 60 GHz systems with spreading 2 and 4. Our results indicate a limited increase in robustness at high CCI situation compared to non spreading cases.