Time and frequency synchronisation in optical wireless OFDM networks

This paper analyses the impact of imperfect synchronisation in optical orthogonal frequency division multiplexing (OFDM) systems utilising intensity modulation (IM) / direct detection (DD). The use of IM/DD technique inherently eliminates the problem of carrier frequency offset. Therefore, time-frequency synchronisation in optical OFDM system reduces to frame synchronisation and sampling clock synchronisation. Sampling clock offset causes irreducible intercarrier interference (ICI) and symbol timing offset. A technique to mitigate the impact of sampling clock offset is proposed in this paper. In the proposed approach, the received signal is oversampled at twice the Nyquist rate and the odd samples are punctured. The symbol timing offset is estimated using pilots and when the symbol timing offset exceeds ±0.5 of sample duration, the puncturing pattern flips between puncturing the odd series and the even series. This in effect, constraints the symbol timing offset within ±1 sample duration. The residual symbol offset can be perfectly corrected using linear phase equaliser. The results show that the proposed method attains a performance comparable to the system that is ideally synchronised provided that the sampling clock offset is lower than 50 ppm.