Many 5G solutions have been proposed, with the goal of either increasing the efficiency of existing resources or providing new radio resources or infrastructure. Device-to-device (D2D) communications are regarded as a promising technique that allows mobile devices to communicate directly with one another without the use of access points or BSs. The implementation of D2D communications in a multi-cellular network environment presents a number of technical challenges that must be addressed. This work focused on interference management, which stands out as a critical and complex issue that must be addressed. In this work, an improved resource allocation algorithm for interference mitigation with improved QoS for cellular and Device-to-Device (D2D) communication was developed. Aheuristic allocation scheme was employed. The system was such that the CUEs were uniformly distributed, and the transmitter (DUE-Tx) and the receiver (DUE-Rx) of each D2D pair were also uniformly distributed in a cluster. Two important metrics namely access rate and D2D throughput gain were used to evaluate the performance and efficiency of the proposed resource allocation scheme. To validate the performance of the developed algorithm, the impact on the D2D throughput gain for different SINR requirement was compared to the result obtained by another research work. Simulations result showed that as the SINR requirement increased, the access rate and D2D throughput gain of the system was reduced. Although the two methods leveraged the greedy heuristic algorithm, the method used in this work increased the achievable throughput by introducing an additional threshold for minimum SINR requirement, such that the throughput was increased as the access rate increased. Comparisons showed that the developed method had a 5.3% improvement over the method by Celik et al (2017). Also, when the maximum distance between the DUE-Tx and DUE-Rx was 100m, the developed method showed about 60.9% improvement.