IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 55, NO. 5, MAY 2007 941 Opportunistic Beamforming and Scheduling for OFDMA Systems Patrick Svedman, Student Member, IEEE, Sarah Kate Wilson, Senior Member, IEEE, Leonard J. Cimini, Jr., Fellow, IEEE, and Björn Ottersten, Fellow, IEEE Abstract—Orthogonal frequency-division multiple access (OFDMA) is an attractive technique for exploiting multiuser di- versity in the downlink of a cellular system. This paper addresses three problems in multiuser diversity for OFDMA systems. First, we propose a way to significantly reduce the amount of channel state information (CSI) feedback without sacrificing performance too much, by selective and adaptive feedback. Second, we propose a way to increase the cell throughput and fairness by appying an opportunistic beamforming scheme to orthogonal frequency-division multiplexing. This beamforming scheme increases the frequency fading rate, which increases the multiuser diversity effect. Thirdly, we deal with the issue of fairness and quality-of-service (QoS) in opportunistic systems by proposing a modified proportional fair (PF) scheduler for OFDMA. Key features in the scheduler are that it incorporates QoS classes into the PF scheduler and that it has a tunable fair- ness level. Extensive simulation results are presented to evaluate the performance of the proposed schemes. The opportunistic beamforming scheme performed well in comparison with several other schemes. The modified PF scheduler was able to give users different QoS, based on their requirements, while still exploiting multiuser diversity. Index Terms—Multiple antennas, multiuser diversity, OFDM, scheduling, wireless system design. I. INTRODUCTION O PPORTUNISTIC systems use adaptive modulation in- stead of power control to achieve the target error rates. By scheduling users with good instantaneous channel conditions, exploiting multiuser diversity, high system throughput can be achieved [1]. In the downlink of an opportunistic system with frequency-selective channels, orthogonal frequency-division multiple access (OFDMA) is suitable because users can be scheduled on orthogonal frequency bands. This enables the exploitation of multiuser diversity in the frequency domain, i.e., users can be scheduled also on their frequency fading peaks [2], [3]. In this paper, we deal with some of the problems with op- portunistic OFDMA. We propose an adaptive reduced-feedback scheme to cope with the significant amount of channel state Paper approved by T. F. Wong, the Editor for Wideband and Multiple Access Wireless Systems of the IEEE Communications Society. Manuscript received October 18, 2004; revised July 30, 2006. This paper was presented in part at the Vehicular Technology Conference, Milano, Italy, May 2004, and in part at the Vehicular Technology Conference, Los Angeles, CA, September 2004. P. Svedman and B. Ottersten are with the KTH School of Electrical Engi- neering, SE-100 44 Stockholm, Sweden (e-mail: patricks@ee.kth.se). S. K. Wilson is with the Department of Electrical Engineering, Santa Clara University, Santa Clara, CA 95053 USA. L. J. Cimini Jr. is with the Electrical and Computer Engineering Department, University of Delaware, Newark, DE 19716 USA. Digital Object Identifier 10.1109/TCOMM.2007.896082 information (CSI) feedback required in a frequency-division duplexing (FDD) opportunistic OFDMA system. Furthermore, we propose an opportunistic beamforming scheme for OFDMA in order to increase the cell throughput and increase fairness. Fairness and QoS guarantees are usually weak points in op- portunistic systems. We propose a modified proportional fair (PF) scheduler for orthogonal frequency-division multiplexing (OFDM) that addresses these weaknesses. The scheduler exploits multiuser diversity, but also tries to meet individual user requirements on bit rates and delays. The fairness of the scheduler is tunable; furthermore, a way to couple the scheduler and the beamformer to help the weakest users is proposed. Opportunistic beamforming uses multiple antennas at the transmitter to increase the temporal fading rate of the individual users [1]. This can help slowly fading users to be scheduled more often. In addition, the fading rate of the intercell in- terference (ICI) is increased, which is called opportunistic nulling. The basic idea of opportunistic beamforming is that the basestation forms a random beam that is changed for each transmission block. Users are then scheduled based on the reported supportable rates. In [1], the concept of using opportunistic beamforming for frequency-selective fading channels using OFDMA is outlined. We extend the idea of [1] by showing how opportunistic beamforming can be applied to OFDMA in practice. Also in [4], the extension of opportunistic beamforming to parallel channels is considered, but without introducing the same randomness in the frequency domain. One of the main problems with FDD opportunistic OFDMA systems is the large amount of feedback required from the users. Because users can be scheduled on different frequency sub- bands, users must feed back measurement information about each subband. We propose to reduce the feedback by grouping adjacent subcarriers into clusters [5] and only feeding back in- formation about the strongest clusters. Additionally, we observe that the suitable feedback rate per user depends on the number of users, and we design the adaptive feedback scheme accordingly. Alternatively, the feedback load can be reduced by feeding back information only from users with channel quality above a cer- tain predefined threshold [6]. Clustered OFDMA and multiuser diversity were also studied in [2] and [3], where the authors showed an increase in spectral efficiency as the number of users grew. We propose the use of identical beamforming weights on all subcarriers within each cluster and independent weights be- tween the clusters. This keeps the correlation high between the subcarriers within the clusters so that feedback of only one value is sufficient, e.g., the supportable rate of the weakest subcar- rier within the cluster. Furthermore, by having different beam- 0090-6778/$25.00 © 2007 IEEE