Device-to-device　(D2D)　communications　can　enhance　the　overall　system　capacity　by　reusing　cellular　spectrum,　which　at　the　same　time　leads　to　severe　interference　to　cellular　connections　and　vice　versa.　In　practice,　even　when　two　wireless　connections　share　the　same　spectrum,　the　interference　between　them　may　not　always　exist　if　there　is　no　conflict　at　packet-level　transmissions.　Thus,　in　this　paper,　we　establish　a　cross-layer　model　for　D2D　communications　underlaying　cellular　network　to　characterize　the　realistic　interference　scenario.　The　closed-form　stable　throughput　region　between　one　D2D　pair　and　one　cellular　link　is　deduced.　Subsequently,　our　model　is　extended　to　a　generalized　scenario　where　multiple　D2D　pairs　share　the　same　resources　with　one　cellular　link,　in　which　the　upper　bound　of　the　stable　throughput　region　is　derived.　As　a　consequence,　the　stable　throughput　regions　are　significantly　enlarged　via　the　proposed　model.