4876 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 8, NO. 9, SEPTEMBER 2009 Adaptive Modulation and Coding Techniques for OFDMA Systems Romano Fantacci, Fellow, IEEE, Dania Marabissi, Member, IEEE, Daniele Tarchi, Member, IEEE, and Ibrahim Habib, Senior Member, IEEE Abstract—The demand for high-speed services and multimedia applications anywhere and anytime has led to the rise of wireless communications. In particular, WiMAX technology is nowadays considered one of the most prominent solutions capable to provide a Broadband Wireless Access (BWA) in metropolitan areas with a simpler installation and lower cost than traditional wired alternatives. This paper deals with the proposal of efficient adaptive modulation and coding techniques to be used in WiMAX based wireless networks, that allow to improve network perfor- mance in the case of Non Line-of-Sight communications, which are typical in urban environments. Through these techniques it is possible to switch the modulation order and coding rate in order to better match the channel conditions, and, hence, obtaining better performance both in terms of error probability and data throughput. Index Terms—OFDMA, adaptive modulation and coding, IEEE 802.16, broadband wireless access, WiMAX. I. I NTRODUCTION H IGH data rate services, multimedia applications and, in general, high quality information streams are nowadays in high demand and will continue for the near future. Wireless systems are considered to be a viable and attractive solution to provide high data rates communications, in particular to mobile users. Recently, the IEEE 802.16 family of standards [1], [2], supported by the WiMAX commercial consortium, provides the specifications for the Physical and Medium Access Con- trol (MAC) layers for a Broadband Wireless Access (BWA) communication protocol. Among several alternatives the IEEE 802.16 standards foresee the use of the Orthogonal Frequency Division Multiplexing (OFDM) in order to mitigate the ad- verse effects of the frequency-selective multi-path fading and efficiently contrast the inter-symbol and inter-carrier interfer- ences (ISI and ICI). In particular, we focus our attention on the Orthogonal Frequency Division Multiple Access (OFDMA) scheme that is the basis of the IEEE 802.16e standard [2]. Manuscript received February 18, 2009; revised May 14, 2009; accepted June 8, 2009. The associate editor coordinating the review of this paper and approving it for publication was N. Kato. R. Fantacci, D. Marabissi, and D. Tarchi are with the Department of Electronic and Telecommunications, University of Florence, Via di S. Marta, 3, 50139 Firenze, Italy (e-mail: {romano.fantacci, dania.marabissi, daniele.tarchi}@unifi.it). I. Habib is with the Department of Electrical Engineering, City College of the City University of New York, New York, NY 10031, USA (e-mail: habib@ccny.cuny.edu). This work has been partially supported by Italian National projects IN- SYEME, under grant number RBIP063BPH, and WORLD, under grant number 2007R989S. Digital Object Identifier 10.1109/TWC.2009.090253 Differently from the classical OFDM technique, OFDMA al- lows to assign each subcarrier independently to the users, thus, enabling a flexible use of resources and making it possible to support nomadic or mobile applications. The multicarrier nature of the OFDMA transmission reaches better results if integrated with adaptive techniques in order to achieve higher efficiency in terms of error rate and throughput. Our aim is to fully exploit the characteristics of the mul- ticarrier transmission and flexibility offered by the OFDMA scheme for optimizing the use of the network resources by assigning adaptively modulation and coding to each data transmission, based on actual channel conditions. Among several resource allocation techniques, Adaptive Modulation and Coding (AMC) allows OFDMA systems to select the most appropriate Modulation and Coding Scheme (MCS) depending on the propagation conditions of the com- munication channel: during good propagation conditions a high order modulation scheme with low coding redundancy is used in order to increase the transmission data rate, while dur- ing a signal fade, the system selects a more robust modulation scheme and a higher coding rate to maintain both connection quality and link stability without increasing the signal power. The use of AMC schemes in wireless communication sys- tems is a topic widely considered and investigated in the recent literature [3]–[12]. In [3] the idea of combining the OFDM technique with adaptive modulation and coding is presented by showing the advantages in terms of overall throughput, while in [4], [5] the principles of AMC were proposed and investigated. Successively, in [6], [7] the problem of link adaptation in OFDM system was introduced by considering jointly subcarriers, modulation and power allocation. Adaptive resource allocation in OFDMA systems was also considered in [8], where a novel technique for balancing fairness and capacity was proposed, and in [9], where an utility based approach was discussed. The use of AMC in WiMAX and OFDMA systems was considered in [10], [11] with special focus on WiMAX systems and services. Finally, a joint analysis of AMC techniques and admission control schemes in OFDMA systems was proposed in [12]. Differently from previous papers [3]–[12], the focus here is on AMC schemes that are compliant with the IEEE 802.16e standard [2] due to the use of the slot basis subcarrier allocation instead of the simple (i.e., individual) subcarrier allocation [8]–[12]. Moreover, a suitable model of the AMC process is also outlined here to make possible an extension of the obtained results to different application scenarios or user 1536-1276/09$25.00 c ⃝ 2009 IEEE