Compensation of Sampling Frequency Offset With Digital Interpolation for OFDM-Based Visible Light Communication Systems

Compared with the traditional wireless communication systems, a visible light communication (VLC) system with intensity modulation and direct detection suffers nonlinearity and bandwidth-limitation of practical light emitting diodes (LEDs), which affect the synchronization and transmission performance of an orthogonal frequency division multiplexing signal. To improve the system spectral efficiency for relatively high capacity, an effective scheme for compensation of a sampling frequency offset (SFO) 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 the Farrow structure is proposed to digitally compensate SFO for the VLC systems. The LED nonlinearity causes extra high-frequency components in the signal spectrum, which may aggravate the aliasing effect when digital compensation of SFO is applied at the receiver. A theoretical study is, therefore, given to the impact of both the LED nonlinearity and SFO-induced intercarrier interference on the SINR based on a second-order polynomial nonlinear LED model. It is shown that the minimum required oversampling rate for achieving the maximum SINR using the fourth-order interpolator is reduced by approximately 15% and 50% compared to the second- and third-order interpolators, respectively. Both numerical and experimental results indicate that the proposed scheme can be used to effectively compensate a local oscillator frequency offset up to ±1000 ppm at a minimum oversampling rate of 1.3 at the receiver.