Iterative Decoding of Virtual Differentially Coded OFDM Systems

This paper describes a virtual differentially coded system without a differential encoder at the transmitter and channel estimates at the receiver. Without the assistance of a differential encoder, a differential decoder detects a linearly multiplexed symbol that is constituted by two transmitted symbols in different times. In the virtual differentially coded system, the multiplexed symbol is regarded as a virtual symbol. In order to resolve the virtual symbol into the two original symbols, extrinsic log likelihood ratio (LLR), which is fed back from the channel decoder, is exploited with the aid of iterative decoding. For the sake of improving the convergence property of iterative decoding, sparse pilot symbols are doped in a stream of transmitted symbols. We demonstrate that the proposed system with a small number of sparse pilot tones is very assistive in terms of frame error rate (FER) performance. Furthermore, we explicitly show that the proposed approach outperforms the conventional differential and coherent detection schemes under the condition of a few sparse pilots.