PMD Tolerant Direct-Detection Optical OFDM System

We show that PMD degrades the transmission performance of an optical OFDM system which is based on direct-detection and single side-band transmission. Maximum PMD tolerance can be achieved applying polarization diversity together with polarization control. Introduction Optical transmission systems deploying orthogonal frequency division multiplexing (OFDM) have gained interest recently. Two principle architectures have been proposed in order to preserve linearity of distortions in the optical domain beyond optoelectrical conversion: A setup using coherent detection was shown to be robust against chromatic dispersion. Transmission over 3000 km SSMF is possible with less than 2 dB OSNR penalty at a bit error ratio of 10 [1]. Extending this approach to a polarization diversity receiver yields a PMD tolerant transmission system [2]. An alternative setup which is less demanding from optical component point of view is based on single side-band (SSB) transmission and direct detection (DD) [3], which also shows chromatic dispersion tolerance. This contribution examines the influence of PMD on the transmission performance of this architecture. It turns out that PMD causes fading which leads to performance degradation. We propose a PMD tolerant extension which uses a polarization diversity receiver. Investigated Setup The basic building blocks of the investigated setup are depicted in Figure 1 [3]. A certain number of subcarriers are modulated using a 4-QAM-constellation occupying the upper half of the upper side-band. Lower sub-carriers are not modulated; the spectral part of negative frequencies is fed by corresponding complex-conjugates. Hence, after inverse discrete Fourier transform, cyclic extension, and digital to analogue conversion we obtain an up-converted, real valued signal s(t). Alternatively, the signal can be generated by up-converting the original complex valued base-band signal in the electrical domain through RF mixing. The resulting signal is modulated onto an optical carrier by a Mach-Zehnder modulator; then the lower side-band of the optical signal is suppressed (see insets of Figure 1). The SSB signal propagates over the optical link and is received through direct detection. Even after squaring there are no beat-products within the used spectrum. In contrast to the architecture based on coherent detection laser phase noise is not an issue. The received signal r(t) is sampled; after cyclic prefix removal and DFT one retrieves distorted symbols Yk,l. Zero-forcing equalization uses estimates of the respective channel coefficients (which are supposed to be perfectly known to the receiver in our simulations)