With increasing needs in cellular network technology, such as high bandwidth requirements for multimedia applications, number of users and limited bandwidth available to cellular network operators, Device to Device (D2D) communication has become important area of research. This Book is about the investigation of Performance Parameters such as outage probability and optimum distance in Device to Device Communication (D2D) and maintaining satisfactory Quality of Service (QoS). Also a technique for improvement of BER performance under frequency selective channels has been proposed.