Investigations on Optimum Roll-off Factor for DFT-Spread OFDM Based SC-FDMA Radio Access in Evolved UTRA Uplink

This paper investigates the optimum roll-off factor value of the spectrum shaping filter in DFT-Spread OFDM based single- carrier frequency division multiple access (SC-FDMA) using the criterion of the maximum achievable throughput considering a cubic metric (CM) in the Evolved UTRA uplink. Simulation results show that the increase in throughput by employing the roll-off factor of greater than zero is small, although a roll-off factor of alpha = 0.16 slightly improves the throughput compared to that for alpha = 0.0 for 16QAM modulation with high coding rates only when there is multiuser interference from/to the adjacent channels. Moreover, in the low received signal energy per-symbol-to-noise power spectrum density ratio (E_s/N₀,) region where repetition coding is necessary, although we observe that alpha = 0.16 achieves higher throughput than alpha = 0.0 for the respective repetition factor (RF) values owing to the lower CM value with alpha = 0.16, the advantage obtained when alpha = 0.16 is concealed by the use of a high RF value and the advantage of alpha = 0.16 is observed only with the highest RF value, i.e., minimum modulation and coding scheme. As a result, we conclude that the benefits to using a high roll-off factor that provides a low CM are minor and subsequently, spectrum shaping with a roll-off factor of greater than zero is not necessary in the average received E_s/N₀ region in cellular environments.