Device-to-Device　(D2D)　communication　is　viewed　as　an　emerging　technology　in　the　fifth　generation　systems　to　fully　explore　the　proximate　gain　residing　in　local　communicating　pairs.　But　the　interference　imposed　to　the　existing　Cellular　links　(C-link)　should　be　addressed　in　the　optimization　for　underlaid　D2D　links　(D-link).　There　is　a　performance　tradeoff　between　Energy　conservation　(EC)　and　the　number　of　links　with　satisfied　Quality-of-service　(QoS)　requirement　or　System　capacity　(SC).　We　aim　to　weight　the　above　tradeoff　for　D-links　by　specifying　it　into　optimizing　EC　on　the　condition　that　SC　is　maximized　and　QoS　requirements　of　C-links　are　fully　guaranteed.　A　three　stage　joint　power　control　and　channel　assignment　mechanism　is　proposed　including　feasibility　check,　SC　maximization　and　greedy　optimization　for　EC.　Thanks　to　numerical　results,　we　illustrate　the　necessity　to　conduct　our　performance　tradeoff　and　observe　the　gain　when　multiple　resource　variables　are　in　elaborate　cooperation.