Comparison of OFDM and Single-Carrier Transmission for Ultra-Low Delay Applications

In this paper, we show that Orthogonal Frequency Division Multiplexing (OFDM) is not well suited for transmission over wireless channels with intersymbol interference (ISI), if a low delay constraint prohibits the use of channel codes with long code words. We compare OFDM with single-carrier transmission for scenarios with very short block lengths and different code rates. The single-carrier receive signal is equalized and decoded using either a Turbo scheme or a so-called matched decoder, which combines channel equalization and decoding and allows for a flexible trade-off between complexity and performance. Our results show that OFDM suffers from severe losses compared to single-carrier transmission if a very low latency—corresponding to a very short block length—is required. This effect is most pronounced for high code rates.