# Resource Allocation for GBR Services in D2D-Enabled Communication
## 1 Introduction  
In device-to-device (D2D) communications, devices in the same vicinity can communicate directly with each other without routing the data path through a network infrastructure, e.g., the Uu-interface in the 3 rd Generation Partnership Project (3 GPP). A device capable of the direct communication is called a DU, and a DU consist of a D2D transmitter (DT) and a D2D receiver (DR). This approach provides a number of advantages. It can reduce the cellular network load because it does not use the cellular network infrastructure. It is possible to spatially reuse uplink frequency resources in the cellular network, thereby increase overall network frequency efficiency. In addition, since it is a short-range communication between devices, the communication delay and the transmission power can be drastically reduced [1]. In order to take advantage of D2D communication, 3 GPP selected D2D technology as a study item in Release 12 in 2011 and conducted a feasibility study in the name of ProSe (Proximity-based Service). In Releases 13 and 14, mission critical enhancement and LTE support for the enhancement of ProSe communication and V2X service were performed. Recently, in Releases 15 and 16, further works have been done to satisfy the high data rate and low latency requirements in 5G systems [2]. For example, in V2V communications, safety-related data is time-critical and D2D can be a perfect solution [3]. Energy-efficient D2D based communication has been studied for the smart grid and Internet of Things (IoT), and to meet the transmission delay requirements needed for real-timeinformation collection [4; 5]. In addition, the problem of reliability during constant bit rate streaming in a real-time streaming network has also been studied [6]. In D2D-enabled communications, a channel can be matched to a CU-DU pair. Depending on the sharing mode, two separate resources can be allocated to a channel using a dedicated mode, or a single resource can be shared in a shared mode. Matching the CU-DU pair and channel is crucial to maximize system performance with the quality of service (QoS) constraints. In Reference [7], an optimal power control and resource allocation scheme were proposed to maximize the sum rates. An optimal mode selection for DUs was additionally considered in [8], and energy efficiency was evaluated as a performance measure in [9]. In these previous works, each CU was pre-assigned a channel with a fixed channel bandwidth. Then, the best DU was matched to each CU and inherently to each channel, to maximize the sum rate. In a shared mode, interference is induced and as a consequence, the channel capacity varies. Accordingly, the required channel bandwidth needs to be determined, to satisfy the required target rate with the experienced channel capacity. Since the received SINR may vary, a proper modulation and coding scheme (MCS) needs to be informed during a scheduling process for successful reception [10; 11]. It is also noted that resource information indicating the amount of resource and its location can be informed and therefore, the channel bandwidth can be adaptively adjusted. The transmit power also needs to be controlled adaptively according to channel bandwidth, as well as channel quality. Guaranteed bit rate (GBR) service is required in many applications, such as real-time gaming and audio/video communications, requiring a minimum data rate and strict latency. In previous works, power control and resource allocation methods were proposed to guarantee QoS requirements, such as SINR or data rate [12; 13; 14; 15; 16; 17; 18; 19]. However, since most of these works allocate a single channel of a fixed bandwidth to one CU, there is still a problem in that some users have a SINR or a data rate below their QoS requirements. In other words, when the received SINR is too low due to high co-channel interference, the fixed channel bandwidth may not be wide enough to guarantee the target data rate. Therefore, it is optimal to manage the channel bandwidth and channel assignment jointly.