Spectrum auctions provide a platform for licensed spectrum users to share their underutilized spectrum with unlicensed users. Existing spectrum auctions either use the protocol interference model to characterize interference relationship as binary relationship, or do not allow the primary and secondary users to share channels simultaneously. To fill this void, we design SPA, a spectrum single-sided auction under the physical interference model, which considers the interference to be accumulative. We prove that SPA is truthful, individually rational, and computationally efficient. Results from extensive simulation studies demonstrate that, SPA achieves higher spectrum utilization and buyer satisfaction ratio, compared with an existing auction adapted for the physical interference model.