Sphere Decoding for Unique Word OFDM

The recently presented UW-OFDM (Unique Word - orthogonal frequency division multiplexing) signaling scheme [1] uses certain subcarriers in frequency domain for redundant symbols instead of data, in order to generate a zero word in the DFT (discrete Fourier transform) interval in time domain. These redundant symbols depend on the data loaded on the other carriers and thus introduce correlation. The resulting linear system model enables sophisticated detectors for data recovery. As the best known maximum likelihood detector for this case, we applied the Sphere Decoding (SD) algorithm to a single antenna UW-OFDM system and evaluated its bit error performance in AWGN (additive white Gaussian noise) and frequency selective environments. Compared to linear receivers, the SD is able to take the correlations on the redundant subcarriers optimally into account and shows an enormous gain. A reduction of the redundant energy by increasing the number of redundant subcarriers improved the bit error performance of UW-OFDM systems with linear data estimators [2] at the price of a lower bandwidth efficiency. In contrast it is found, that a receiver based on Sphere Decoding is able to optimally exploit the excess redundant energy. The SD based system therefore shows its best performance at maximum bandwidth efficiency.