IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 62, NO. 9, NOVEMBER 2013 4539 Green Resource Allocation Schemes for Relay-Enhanced MIMO-OFDM Networks Tain-Sao Chang, Kai-Ten Feng, Member, IEEE, Jia-Shi Lin, and Li-Chun Wang, Fellow, IEEE Abstract—This paper studies the problem of joint allocation of subchannel, transmission power, and phase duration in relay- enhanced bidirectional multiple-input–multiple-output orthogo- nal frequency-division multiplexing (MIMO-OFDM) networks. The goal of resource allocation is to minimize transmission energy consumption in networks with multiple decode-and-forward relay stations (RSs) under the data rate constraints of user equipment (UE). The challenges of this resource allocation problem arise from the complication of multiple-phase assignments within a sub- channel since the RS can provide an additional transmission path from the base station to the UE. Existing research does not fully take into account all of the influential factors to achieve feasible resource allocation for relay-enhanced MIMO-OFDM networks. Green resource allocation (GRA) schemes with reduced computa- tional complexity are proposed in this paper to develop joint allo- cation of subchannel, power, and phase duration for the UE with the consideration of direct and two-hop communications. Both the separate-downlink (DL)-and-uplink (UL) and mixed-DL-and-UL relaying assignments and the linear block diagonalization (LBD) technique are adopted to obtain the solutions for the proposed GRA schemes. Simulation results show that the proposed GRA schemes can provide comparably better energy conservation with the consideration of quality-of-service (QoS) support. Index Terms—Energy conservation, linear block diagonaliza- tion (LBD), multiple-input multiple-output orthogonal frequency- division multiplexing (MIMO-OFDM), relay station (RS), resource allocation. I. I NTRODUCTION R ESEARCH on a relay-enhanced communication was first published in [1]. In the relay-based cooperative com- munication, the relay station (RS) is introduced to provide an alternative path between the base station (BS) and user equipment (UE). Recently, cooperative communications have attracted significant attention due to their potential benefits of providing wide-range coverage to the UE and achieving spatial diversity gains. Furthermore, the orthogonal frequency-division multiplexing (OFDM) system is a key transmission technology for next-generation wireless communication systems, including Manuscript received September 9, 2012; revised January 20, 2013 and March 28, 2013; accepted May 2, 2013. Date of publication June 3, 2013; date of current version November 6, 2013. This work was supported in part by the Aim for the Top University and Elite Research Center Development Plan under Grant NSC 101-2628-E-009-003 and Grant NSC 101-2219-E-009-027; by the MediaTek Research Center, National Chiao Tung University; and by Telecommunication Laboratories, Chunghwa Telecom Co. Ltd., Taiwan. The review of this paper was coordinated by Prof. Y. Zhou. The authors are with the Department of Electrical and Computer Engineer- ing, National Chiao Tung University, Hsinchu 300, Taiwan (e-mail: ktfeng@ mail.nctu.edu.tw). 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/TVT.2013.2265779 the worldwide interoperability for microwave access (WiMAX) [2] and the downlink (DL) of the Third Generation Partnership Project Long-Term Evolution (LTE) [3]. The OFDM system divides the wideband channel into numerous narrowband sub- channels to provide high spectral efficiency and mitigate the effects of multipath propagations. The use of an orthogonal frequency-division multiple-access (OFDMA) technique in the OFDM system can achieve multiuser diversity by adopting opportunistic scheduling, which appropriately allocates the subsets of subchannels to individual UE. Moreover, the multiple-input–multiple-output (MIMO) system has been ex- tensively studied in recent years. An intrinsic advantage of the MIMO system is that the system can support higher data rates under the same transmission power and bit error rate (BER) requirements as a single-input–single-output (SISO) system. On the other hand, the MIMO system requires less transmission power than a SISO system under the same data rate requirement. Owing to severe path loss within a channel, it is important to provide efficient resource allocation for the relay-enhanced MIMO-OFDM systems to improve throughput, energy consumption, and outage performances. Resource al- location of the relay-enhanced MIMO-OFDM system benefits from spatial and frequency diversity and cooperative multiuser diversity due to the space and frequency-selective channels. In this paper, an RS is considered to be half-duplex, and each subchannel is partitioned into two transmission phases in the time domain for the relay-enhanced network, including the first phase to allocate the transmission between the BS and the RS and the second phase for the RS and UE. Existing research work has been conducted to investigate the resource allocation and power control mechanisms for the OFDM [4], [5] and OFDMA [6]–[9] networks. However, these schemes cannot be directly applied to the networks with the assistance of an RS. The problems of resource allocation for a decode-and-forward (DF) relaying OFDM system to maximize throughput are studied in [10]–[13]. In [10], power allocation under total power constraints for the BS and the RS and subcarrier pairing are studied. However, subcarrier pairing and power allocation are separately optimized in this work. The optimal joint subcarrier pairing and power allocation with an individual power constraint for the BS and the RS is considered in [11]. In [12], joint subcarrier pairing and power allocation to maximize the weighted sum rate with both total power and indi- vidual power constraints is investigated. However, these works assumed the existence of a single RS in the network. Multiple RSs are considered in [13], in which allocation of power, sub- channel, and time duration for multiple UE is studied. Resource allocation strategies for cooperative OFDM systems with 0018-9545 © 2013 IEEE