Coherent Optical DFT-Spread OFDM

We consider application of the discrete Fourier transform-spread orthogonal frequency-division multiplexing (DFT-spread OFDM) technique to high-speed fiber optic communications. The DFT-spread OFDM is a form of single-carrier technique that possesses almost all advantages of the multicarrier OFDM technique (such as high spectral efficiency, flexible bandwidth allocation, low sampling rate, and low-complexity equalization). In particular, we consider the optical DFT-spread OFDM system with polarization division multiplexing (PDM) that employs a tone-by-tone linear minimum mean square error (MMSE) equalizer. We show that such a system offers a much lower peak-to-average power ratio (PAPR) performance as well as better bit error rate (BER) performance compared with the optical OFDM system that employs amplitude clipping. 1. Introduction The high-throughput data transmission over long-haul fiber optic systems is of considerable current interest. To maximize the spectral efficiency, polarization multiplexing and coherent detection have become the key enabling technologies for high-speed fiber optic communication systems [1]. However, physical impairments such as the chromatic dispersion (CD), the polarization mode dispersion (PMD), and the polarization dependent loss (PDL) become more severe as the bandwidth and data rate increase. The orthogonal frequency-division multiplexing (OFDM) technique has been widely adopted to cope with the frequency-selective fading of multipath channels in wireless communications, and it has been recently introduced to fiber optic systems for high-speed data transmission [2]. In OFDM systems, the frequency-domain equalization is employed with a single-tap equalizer at each tone, which significantly reduces the computational complexity compared with the time-domain equalization in single-carrier systems. However, one major disadvantage of the OFDM system is the high peak-to-average power ratio (PAPR). To address this issue, the DFT-spread OFDM has been developed as an alternative wireless access technique, and it has been adopted as the uplink air interface of 3rd generation partnership project long-term evolution (3GPP LTE) [3]. In [4] it is observed that the impact of nonlinearity on a link with periodical dispersion compensation is significantly larger than that on a link without inline dispersion compensation, which makes the application of OFDM to the existing infrastructure questionable. In addition, it is also noted that, in a periodic dispersion map, reducing PAPR at the transmitter might significantly improve the