Visible light communications: Fast-orthogonal frequency division multiplexing in highly bandlimited conditions

Abstract

In this paper we propose, for the first time, fast orthogonal frequency division multiplexing (FOFDM) for visible light communications (VLC) systems. VLC systems often exhibit highly band-limited system responses and as such supporting high transmission speeds is a key challenge. FOFDM makes use of an inverse discrete cosine transform (IDCT) to generate the time domain symbols, as opposed to the inverse fast Fourier transform (IFFT) used in OFDM. This offers several advantages for VLC in particular, because sacrificing complex modulation formats in favour of real ones such as pulse-amplitude modulation (PAM) enables reduction of the subcarrier spacing to 1/2T , where T is the symbol period, whilst maintaining orthogonality. This results in a bandwidth saving of 50% in comparison to OFDM, whilst maintaining an equivalent spectral efficiency. Hence in this work, we examine the bit error rate performance of F-OFDM in comparison to conventional OFDM with an equal number of subcarriers and equivalent spectral efficiency for a number of band-limited conditions as a function of the energy-per-bit to noise spectral density ratio (E b /N0). We demonstrate that due to the 50% bandwidth savings, F-OFDM outperforms OFDM in band-limited conditions, because the impact of the attenuation caused by the band-limitation on each subcarrier is reduced. Therefore, we show that FOFDM results in lower electrical power penalty relative to conventional OFDM for a given set of band-limitation conditions, while maintaining equivalent spectral efficiency.

Non‐linear modelling and distortion compensation in optical Fast‐orthogonal frequency division multiplexing systems

“…where y(n) is the baseband received signal, a k are the model coefficients, and K is the polynomial order. In (15), we include both odd-and even-order non-linear terms and set K to 3.…”

“…2PAM-based training sequences (TS) are used for both post-distortion training and frequency domain channel estimation. In the post-distorter model of Equation (15), K is set to 3. The GMP model parameters in Equation ( 8) are as follows:…”

“…This is achieved, respectively, by either compressing the frequency spacing between the subcarriers down to 1/2 T , where T is the symbol duration, or sampling faster in comparison to conventional OFDM. The reduction in inter‐carrier spacing is beneficial in frequency selective links as it results in a flatter response per subcarrier [15]. In Fast‐OFDM, the subcarriers are modulated with real data symbols using inverse discrete cosine transform (IDCT), and hence there is no need for Hermitian transpose at the transmitter.…”