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. 

Key words: Fiber optic communications, DFT-spread OFDM, polarization- division multiplexing (PDM), PAPR, linear MMSE. 

## 1 Introduction 

The high-throughput data transmission over long-haul fiber optic systems is of considerable current interest. To maximize the spectral efficiency, polar- ization 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 polariza- tion mode dispersion (PMD), and the polarization dependent loss (PDL), become more severe as the bandwidth and data rate increase. The orthog- onal 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 op- tic 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 com- pared with the time-domain equalization in single-carrier systems. However, 

