IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 6, NO. 3, MARCH 2007 1083 Adaptive Resource Allocation and Capacity Comparison of Downlink Multiuser MIMO-MC-CDMA and MIMO-OFDMA Ernest S. Lo, Student Member, IEEE, Peter W. C. Chan, Vincent K. N. Lau, Senior Member, IEEE, Roger S. Cheng, Member, IEEE, K. B. Letaief, Fellow, IEEE, Ross D. Murch, Senior Member, IEEE, and Wai Ho Mow, Senior Member, IEEE Abstract— In this paper, we examine and compare the potential maximum sum capacity of downlink multiple-input-multiple- output orthogonal frequency division multiple access (MIMO- OFDMA) and multiple-input-multiple-output multicarrier code division multiple access (MIMO-MC-CDMA) in a single-cell multiuser environment with channel side information at the transmitter, with and without a fairness constraint. The resource allocation is formulated as a cross-layer optimization framework and optimal power allocation and user selection algorithms are proposed for both scenarios. We find that for delay-sensitive applications, where fairness is imposed, the performance gain of OFDMA over MC-CDMA is quite large at moderate path loss exponents and number of antennas. However, for delay- insensitive applications, the benefits of OFDMA over MC-CDMA are significantly reduced when the path loss exponent or the number of antennas is large. Index Terms— Adaptive resource allocation, cross-layer design, fairness, MC-CDMA, MIMO, multiuser capacity, multiuser di- versity, OFDMA. I. I NTRODUCTION O RTHOGONAL frequency division multiplexing (OFDM) is tremendously gaining in popularity nowadays as a promising technology for supporting high- speed data transmission in multipath fading environments. In future generation broadband wireless networks, such as Beyond 3G (B3G) systems [1], several prevalent candidates for multiple access technology are built on the OFDM structure. One of them is multicarrier code division multiple access (MC-CDMA) [2]–[5], which has garnered significant attention in China and Europe [1], [6] due to the success of CDMA in existing 3G networks. In contrast to direct- sequence CDMA (DS-CDMA), the codes separating different users or services are spread over the frequency domain across subcarriers instead of the time domain. Another potential candidate is orthogonal frequency division multiple access Manuscript received July 4, 2005; revised June 6, 2006; accepted August 24, 2006. The editor coordinating the review of this paper and approving it for publication was M. Saquib. This work was supported in part by the Hong Kong RGC grant HKUST7160-04E. E. S. Lo, V. K. N. Lau, R. S. Cheng, K. B. Letaief, R. D. Murch, and W. H. Mow are with the Hong Kong University of Science and Technology, Hong Kong (e-mail: eeern@ece.ust.hk; eeknlau@ece.ust.hk; eecheng@ece.ust.hk; eekhaled@ece.ust.hk; eermurch@ece.ust.hk; eewhmow@ece.ust.hk). P. W. C. Chan was with the HKUST and is now with the Hong Kong Applied Science and Technology Research Institute, Hong Kong (e-mail: peter@ee.ust.hk). Digital Object Identifier 10.1109/TWC.2007.05510. (OFDMA), which was originally proposed for cable TV (CATV) networks in Europe [7] and has been recently adopted in the IEEE 802.16 WirelessMAN system [8]. Unlike MC-CDMA, users are separated in different subsets of subcarriers with no spreading. In this paper, we consider multiuser adaptive resource allocation in the downlink of both systems and investigate their maximum average sum capacities with different path loss, number of users, number of antennas and by taking the effect of fairness into consideration. Single-cell scenario, or equivalently, multi-cell with white interference, is studied. Such scenario can be treated as a reference and can provide insights to other composite problems. Although OFDMA is generally known to be superior to CDMA-based systems when link adaptation is applied in a single-antenna network, to the best of our knowledge, the actual capacity difference in a practical cellular system and the effect of multiple antennas are not well understood and justified yet in the literature. It is also interesting to explore how the capacities are affected when cross-layer adaptation incorporating multiple-access (MAC) layer scheduling and fairness are considered. Over the past few years, both OFDMA and MC-CDMA have been under rigorous research in different areas. OFDMA, also known as multiuser OFDM, was investigated for a single- antenna network in [9] with the objective of minimizing the to- tal power while satisfying users’ rate requirements. Multiuser OFDM was extended to the multiple-input-multiple-output (MIMO) [10] configuration in [11]–[13] where spatial multi- access was allowed. In [11], subcarrier reuse was achieved by transmit zero-forcing processing and the downlink system capacity was then maximized by an optimal power allocation algorithm without any fairness consideration. In [12] and [13], spatial multi-access was achieved by carefully selecting spa- tially separable users for each subcarrier and the downlink capacity maximization and uplink total power minimization were considered, respectively, with heuristic and suboptimal algorithms provided. In our system, subcarrier reuse is not allowed within the cell, which is attractive in practice due to its simple structure and insensitivity to the channel estimation errors of other users. Fairness is also considered so as to avoid any possible resource monopolization by a small group of users. The error rate comparison between OFDMA and MC- CDMA was studied in [14] for a single-antenna coded system. 1536-1276/07$25.00 c  2007 IEEE