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(Qo S)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 Qo S requirements of C-links are fully guaranteed.A threestage 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. 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 communications pairs. But the interference imposed on 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 weight the above tradeoff for D-links by specifying it to optimize EC on the condition that SC is maximized and Qo S requirements of C-links are fully guaranteed. A threestage 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.