Digital Compensation of Sampling Frequency Offset for OFDM-based Visible Light Communication Systems

Sampling frequency offset (SFO), which is due to mismatch of sampling clocks between transmitter and receiver oscillators, is a key factor affecting transmission performance of orthogonal frequency division multiplexing (OFDM) signal in visible light communication (VLC) systems with bandwidth-limited light emitting diodes (LEDs). To improve the system spectral efficiency for a relatively high capacity, an effective scheme for SFO compensation is required to achieve a high signal to interference plus noise ratio (SINR). From the practical system design point of view, a fourth-order piecewise polynomial interpolator using Farrow structure is proposed to digitally compensate SFO for VLC systems. Nonlinearity of LEDs causes extra high frequency components beyond the OFDM signal spectrum, which may aggravate the aliasing effect when digital compensation of SFO is applied. Theoretical study is thus given to the impact of both the LED nonlinearity and SFO-induced inter-carrier interference (ICI) on SINR of the received OFDM signal. Both numerical and experimental results show that the proposed scheme can be applied to effectively compensate a local oscillator frequency offset up to +/- 1000ppm in an OFDM-based VLC system at a minimum oversampling rate of 1.3 at the receiver.