IEEE SIGNAL PROCESSING LETTERS, VOL. 21, NO. 5, MAY 2014 531 Downlink Resource Reuse for Device-to-Device Communications Underlaying Cellular Networks Daohua Zhu, Student Member, IEEE, Jiaheng Wang, Member, IEEE, A. Lee Swindlehurst, Fellow, IEEE, and Chunming Zhao, Member, IEEE Abstract—The full potential of Device-to-device (D2D) com- munication relies on efficient resource reuse strategies including power control and matching of D2D links and cellular users (CUs). This letter investigates downlink resource reuse between multiple D2D links and multiple CUs. Our goal is to achieve a network utility enhancement for D2D communication while ensuring the QoS of the CUs. Despite the combinatorial nature of the problem and the coupled power constraints, we characterize the optimal D2D-CU matching as well as their power coordination, and propose an efficient algorithm to jointly optimize all D2D links and CUs. The proposed downlink resource reuse strategy shows a superiority over existing D2D schemes. Index Terms—Device-to-device communication, joint optimiza- tion, quality of service, resource reuse strategy. I. INTRODUCTION D EVICE-TO-DEVICE communication is a promising technique to improve spectral utilization by supporting direct communication between two closely located users which would otherwise have to be relayed by a base station (BS) in traditional cellular systems. In addition to high spectral efficiency, D2D communication also enjoys several advantages in terms of data rate, latency and power consumption, etc. It is worth noting that the advantage of D2D communication in enhancing physical layer security has also been investigated in [1]. Therefore, D2D techniques have been proposed to be incorporated into current and future wireless networks [2]. In cellular systems, D2D links may reuse either uplink or downlink resources of CUs for direct communications. Flex- ible and efficient resource reuse (RR) schemes are crucial for Manuscript received January 05, 2014; accepted February 22, 2014. Date of publication February 28, 2014; date of current version March 06, 2014. This work was supported by the 973 Program (2013CB336600 and 2013CB329203), National Natural Science Foundation of China (Grant 61201174), the Natural Science Foundation of Jiangsu (Grant BK2012325), the Fundamental Research Funds for the Central Universities, and the China Scholarship Council. The as- sociate editor coordinating the review of this manuscript and approving it for publication was Prof. Xin Wang. D. Zhu is with the National Mobile Communications Research Laboratory, Southeast University, Nanjing 210096, China, on leave as a visiting student in the Department of Electrical Engineering and Computer Science, University of California, Irvine, CA 92697-2625 USA (email: zhudaohua@seu.edu.cn). J. Wang and C. Zhao are with the National Mobile Communications Research Laboratory, Southeast University, Nanjing 210096, China (email: jhwang@seu. edu.cn; cmzhao@seu.edu.cn). A. L. Swindlehurst is with Center for Pervasive Communications and Computing, University of California, Irvine, CA 92697-2625 USA (email: swindle@uci.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LSP.2014.2309143 underlay D2D communication and have thus received much at- tention [3]–[6]. In particular, [3] studied the sum rate maximiza- tion of one D2D link and one CU, while [4] proposed an inter- ference control scheme for one D2D user with multiple CUs. The RR problems for multiple D2D links and CUs were con- sidered in [5], [6], but in these papers each D2D link can only reuse one CU resource. Most existing works have (e.g. [3]–[5]) considered uplink re- source reuse for the sake of asymmetric uplink and downlink service loads. However, the near-far effect may cause strong in- terference to CUs when D2D links are close to a BS. Therefore, downlink resource reuse is also very important. D2D communi- cation is basically a complement to traditional cellular commu- nication. Hence, CUs generally have higher priority and their QoS must be properly protected [2], [3]. We consider multiple D2D links that reuse the downlink re- sources of multiple CUs in a cellular network. We formulate the RR design as a generic utility maximization problem for the D2D links with QoS constraints for the CUs. Each D2D link is allowed to reuse resources of multiple CUs and thus our result is more general than [5], [6]. Our goal is to jointly optimize the D2D-CU matching and power control of both D2D links and CUs so that the network utility is maximized and the service quality of CUs is protected. The formulated RR problem with QoS constraints is a mixed integer program and is not convex. The power constraint at the BS also presents another challenge in contrast with the up- link RR designs in [3]–[5]. We overcome these difficulties by transforming the original problem into a more tractable form, based on which we characterize the optimal power control and D2D-CU matching. Then, we propose an efficient algorithm to jointly optimize the D2D links and CUs, and achieve the op- timal RR solution. The superiority of the proposed RR scheme is verified by numerical results. II. SYSTEM MODEL AND PROBLEM STATEMENT Consider a hybrid single-cell network as shown in Fig. 1, con- sisting of orthogonal downlink CUs (each occupies a dedi- cated resource block (RB)) and D2D links. Let indicate whether D2D link reuses the RB of CU . To facilitate manipulation and billing of D2D communications, we assume that each CU’s RB can be reused by at most one D2D link, i.e., . A similar constraint was also adopted in [5] and [6]. We introduce the normalized (by the noise power) gains , , , and to represent the transmission channel between the BS and CU , the transmission channel between the D2D transmitter (TX) and receiver (RX) for D2D link on the RB 1070-9908 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.