Unique Word Prefix OFDM (UW-OFDM): An Overview

Mario Huemer^* and Johannes Huber^*

In unique word OFDM (UW-OFDM), cyclic prefixes (CP) of usual OFDM are replaced by a deterministic sequence, the unique word (UW), for conversion of linear convolution due to a dispersive channel into cyclic convolution. This fixed UW may be employed for synchronization and/or channel estimation purposes etc. at the receiver side. In order to generate space for the UW, a block of zeros is created in time domain by means of appropriately loading of redundant subcarriers. By this, a systematic complex number Reed Solomon (RS) code construction is inherently introduced along the subcarriers which additionally can be exploited at the receiver by means of Bayesian linear data estimation (LMMSE), decision-directed methods (SIC, Noise Prediction), or sphere decoding. A remaining problem in the UW-OFDM concept is the fact that the energy on the redundant subcarrier symbols contributes disproportionately to the average symbol energy. A method for reduction of this redundant energy is to spread the redundant energy over all subcarrier symbols by which a nonsystematic complex number RS code construction is introduced. After a discussion of the concepts of systematic and nonsystematic coded UW-OFDM and appropriate data estimation schemes the peak to average power ratio (PAPR) and the peak to minimum ratio (PMR) problems are addressed and methods for the reduction of these measures are discussed. Numerical results and comparisons to usual CP-OFDM are presented for the example 802.11a WLAN standard which illustrate the benefits of UW-OFDM.

Mathematics Subject Classification: 94A05 94A14

Keywords: OFDM, Unique Word, RS Code, Reed Solomon Code

Minisymposion: Unique Word OFDM