1494 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 57, NO. 3, MAY2008 Adaptive Subcarrier Allocation in Synchronous Reverse Links of a Multicarrier CDMA System with Time and Frequency Spreading Ryan Caldwell, Student Member, IEEE, and Alagan Anpalagan, Senior Member, IEEE Abstract—Multicarrier code-division multiple access (CDMA) with time and frequency spreading has been recently considered as a candidate for fourth-generation (4G) wireless systems. This signaling scheme simultaneously utilizes code spreading in the time and frequency domains to simultaneously improve frequency diversity and minimize multiuser access interference. As a result, it is capable of outperforming multicarrier CDMA systems that employ 1-D spreading. In this paper, a novel adaptive subcar- rier allocation algorithm is developed for multicarrier CDMA with time and frequency spreading to improve the overall bit error rate (BER) performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics while simultaneously reducing the amount of interference caused to other users. The proposed algo- rithm is shown to provide a performance improvement, ranging from 1.5 dB with 2 × 16 (time × frequency) and spreading to 7 dB with 16 × 2 (time × frequency) spreading. The algorithm is also shown to maintain or improve the BER floor for each spreading configuration. It is concluded that at higher and lower levels of E b /N o , a higher frequency- and time-domain spreading should be, respectively, employed to improve BER performance. Furthermore, the E b /N o threshold level to switch between time and frequency spreading for the analyzed system is found to be 2.5 dB. Index Terms—Adaptive subcarrier allocation, multicarrier code-division multiple access (CDMA), synchronous reverse links, time- and frequency-domain spreading. I. I NTRODUCTION T HE fourth generation (4G) of wireless systems is intended to support high-throughput applications such as streaming video, multimedia, and Internet access. These services gener- ally require throughput in the megabit-per-second range. To support these high data rates, the channel bandwidth for 4G is as high as 100 MHz in the forward link and 20 MHz in the reverse link. To accommodate this large bandwidth, several 4G systems that utilize multicarrier modulation to minimize the intersymbol interference that occurs when transmitting through wideband wireless channels have been proposed. In particular, multicarrier direct-sequence code-division mul- tiple access (MC-DS-CDMA) has been proposed [1], where Manuscript received October 4, 2005; revised August 14, 2006, August 11, 2007, and September 10, 2007. This work was supported by the Natural Sciences and Engineering Council of Canada (NSERC). The review of this paper was coordinated by Prof. Z. Wang. R. Caldwell is with ZTE Canada, Toronto, ON M2J 4V6, Canada. A. Anpalagan is with Ryerson University, Toronto, ON M5B 2K3, Canada. 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.2007.909280 DS-CDMA waveforms are transmitted over orthogonal subcar- riers. This transmission method is capable of providing multiple access with low interference by employing user-specific spread- ing sequences with low cross correlation. Another proposed 4G architecture is multicarrier CDMA (MC-CDMA) [2]–[4], where spreading occurs in the frequency domain. Each bit is copied to several parallel subcarriers and modulated with chips from the frequency-domain spreading sequence to pro- vide orthogonality between users. Since each bit is transmitted over several independently faded subcarriers, performance is improved due to frequency diversity. The drawback of this approach, however, is that the frequency-selective channel de- stroys the orthogonality between spreading sequences, which may cause high levels of multiuser access interference (MAI). A recently introduced variant of multicarrier CDMA simul- taneously combines time- and frequency-domain spreading. This technology, called multicarrier CDMA with time and frequency spreading in this paper, has been shown to improve performance over MC-CDMA and MC-DS-CDMA systems [5], [6]. This system benefits from frequency diversity while minimizing the amount of MAI by employing time-domain spreading. Further improvements can also be achieved by flexibly controlling the time- and frequency-domain spreading factors based on the fading characteristics and channel load [15]–[17]. Adaptive subcarrier allocation has recently become an active research area for multicarrier systems. Several algorithms have been proposed for MC-CDMA. In [8], the subcarriers are di- vided into small groups and separated throughout the spectrum. This is shown to minimize MAI and maximize gains from fre- quency diversity. In [9], the subcarriers are adaptively assigned based on the fading process, whereas [10] examines the effect of equalizing the interference in each subcarrier group. Adap- tive subcarrier allocation is also effective for MC-DS-CDMA. In particular, forward link adaptation algorithms are devel- oped in [12] and [13], which select subcarriers based on the fading gains. In [14], a reverse link algorithm is proposed, which employs the water-filling principle to assign users to subcarriers with a favorable signal-to-interference-plus-noise ratio (SINR), while simultaneously minimizing the interference to other users. In this paper, we develop a novel low-complexity adaptive subcarrier allocation algorithm for multicarrier CDMA systems with time and frequency spreading to improve the bit error rate (BER) performance for all spreading factors. To the best of the authors’ knowledge, this is the first paper that adaptively 0018-9545/$25.00 © 2008 IEEE