Device-to-Device (D2D) underlaying cellular network is considered to be a promising resource reuse technique. While many literatures have been investigating the air-interface performance of D2D in cellular networks from the physical and/or media access control (MAC) layers, we in this paper introduce factors about contents of information streams (upper layer factors) into the system model and conduct a cross-layer study. To better model the upper layer factors, a new concept called information correlation is proposed and formulated, which can be used as a reference to measure the importance of contents for potential D2D users when we design particular schemes. The proposed model quantitatively estimates users' service demands from the contents perspective to further differentiate user resource allocation, for improving the system performance more effectively with better user experience. A test model and several application examples for information correlation are provided, with simulation carried out for a LTE-based scenario, which corroborates a significant performance gain over traditional schemes.