Single-carrier frequency domain equalization: A focus on wireless applications

The driving force in today's wireless market is the increasing demand for wireless multimedia and interactive Internet services. OFDM has been widely accepted as a viable solution for such high-speed broadband applications. In this article, we have attempted to present a comprehensive overview of a promising alternative solution, SC-FDE, which has been historically shadowed by OFDM. Although the basic ideas behind SC-FDE can be traced back to Walzman and Schwartz's [2] work on adaptive equalizers in 1973, the recent surge of interest in SC-FDE was subsequent to the work of Sari et al. [6]. SC-FDE enjoys a comparable complexity to OFDM due to the similar transceiver architecture based on efficient FFT/IFFT operations. Owing to the single-carrier implementation, SCFDE also avoids the inherent drawbacks of OFDM such as amplifier nonlinearities, carrier frequency offsets, and phase noise. OFDM is commonly used in practice in conjunction with coding and/or adaptive modulation. The comparative performance analysis of SC-FDE, coded OFDM, and adaptive OFDM schemes reveals that SC-FDE achieves comparable (or even better in some scenarios) performance compared to its OFDM counterpart. Several aspects in the practical implementation of SC-FDE have been further studied in the recent years including FD channel estimators, non-linear equalizers, training sequence design, interference rejection etc. These recent studies have also successfully integrated SC-FDE in STC and spatial-multiplexing systems leveraging the potentials of MIMO communications. The literature on SC-FDE has reached to a certain maturity; however, there are still several open research areas for future work. These include: ■ The use of SC-FDE techniques in future-generation multi-band cognitive wireless systems. The interest in this topic is due to the fact that SC-FDE techniques are less sensitive to RF impairment than OFDM; for this reason, they can applied to multiband transmissions exploiting cognitive radio principles [52]. ■ The performance analysis of direct sequence CDMA (DS-CDMA) systems employing SC-FDE and the comparison of such systems with MC-CDMA techniques. Note that SC-FDE techniques in DS-CDMA systems should offer a substantially simpler solution than traditional TD CDMA receivers. ■ The design of noncoherent (and in particular differential) SC-FDE receivers not requiring the knowledge of CSI. Their performance should be compared with that of both channel-estimated-based and adaptive SC-FDE receivers under a variety of operating conditions.