###### Abstract  
The Device-to-Device (D2D) communication introduced in 3 GPP's Release 12 is a promising option for the accommodation of increasing traffic demand and the alleviation of core networks' loading. The D2D communication mode prevails in scenarios where mobile users in proximity form a communication group sharing the same interest in digital content. A user can download intended content from peers in the same communication group rather than from the Internet via the base station. This article addresses the routing issue within D2D communication groups. It is, in effect, a path selection problem. We define a utility function considering both delay and throughput. The path selection problem can be formulated as an NP-hard optimization problem. A Bidirectional and InCremental A-star (BICA*) algorithm incorporating the concept of bidirectional search and lifelong planning is developed to tackle the NP-hard optimization problem. Simulations reveal that the proposed approach outperforms existing ones in terms of less delay, higher throughput, and higher satisfaction ratio. The Greedy approach, Two-Stage Relay Selection (TSRS), and the standard A* algorithm were included in the comparative study. The throughput improvements of the proposed scheme are up to 23% and 46.5% compared to TSRS and Greedy, respectively. The proposed scheme possesses the lowest delay and the highest satisfaction rate, among others. With less computational time, the proposed BICA* is more responsive than the standard A* in dynamic environments. * [table]cappositionposition=top  
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base station. The D2D communication mode is designed for direct communications with nearby User Equipment (UE) without the intervention of base stations. It implies a higher data rate, a better spectrum efficiency, and a lower base station loading. The idea of D2D communication brings about 3 GPP ushering in Proximity Service (ProSe). There are different approaches to the implementation of D2D communication, including Wi-Fi Direct, Bluetooth, and LTE Direct. We focus on D2D communication with LTE Direct in this paper. Figure 2 illustrates the difference between regular cellular communication and LTE D2D communication. With LTE D2D, UEs can communicate with each other directly. The base station is only responsible for the coordination. LTE D2D has great potential for many reasons, including lower base station loading, lower power consumption, improved resource utilization, reduced delay, and improved QoS (Quality of Service) support [4]. There are essential issues in LTE D2D, such as device discovery, mode selection, resource allocation, and group communication. Device discovery is the process of finding matched devices. There are two different device discovery approaches, distributed and centralized approaches [5]. UEs initiate the former, and the latter is controlled by the Mobility Management Entity (MME), the Packet Data Network Gateway (PDN-GW), and the base station. LTE has two different operation modes, namely cellular mode and D2D mode. The D2D mode is further classified into the dedicated mode and reuse mode. These two modes differ in the way resource blocks are allocated. Cellular UEs and D2D UEs in reused mode share the same resource blocks. On the other hand, in dedicated mode, UEs use resource blocks exclusively assigned to them. Which mode should be in use  
Figure 1: An example environment for D2D communication mode. Figure 2: An illustration for cellular and D2D communication. (**a**) Regular cellular communication. (**b**) LTE D2D communication. leads to the resource allocation problem, which is a tradeoff between resource utilization and interference. Intra-group communication (group communication for short) is the primary scenario in this study. As shown in Figure 1, nearby UEs can form a group and are capable of group communication with each other. A UE in a group can access content possessed by other group members via group communication.