Figure 1. Frequency interleaved DDO-OFDM system Frequency Interleaved Directly Detected Optical OFDM for Next-Generation Optical Access Networks Lenin Mehedy, Masuduzzaman Bakaul, Ampalavanapillai Nirmalathas NICTA Victoria Research Laboratory Department of Electrical and Electronic Engineering, The University of Melbourne Victoria, 3010, Australia Lenin.Mehedy@nicta.com.au, Masuduzzaman.Bakaul@nicta.com.au, nirmalat@unimelb.edu.au Abstract— We theoretically analyze and demonstrate that spectral efficiency of a conventional direct detection based optical OFDM system (DDO-OFDM) can be improved significantly using frequency interleaving of adjacent DDO- OFDM channels in future optical access systems. I. INTRODUCTION Optical orthogonal frequency division multiplexing (O- OFDM) brings the benefit of electronic equalization and robustness against multi-path fading of legacy wireless- OFDM systems into the optical domain to combat against fiber impairments, such as chromatic dispersion and polarization mode dispersion (PMD) and achieved impairments-tolerant ultra high speed optical systems [1]. Depending on the detection mechanisms, O-OFDM systems can be broadly categorized into two sub-groups namely, coherent O-OFDM (CO-OFDM) and directly detected (incoherent) O-OFDM (DDO-OFDM). Among them, CO- OFDM systems found to be more complex and expensive, as they require additional signal conditioning devices both in the transmitting and receiving ends [2]. On the other hand, a DDO-OFDM system offers simpler transmitter and receiver architectures, therefore has the potential to be used in next- generation optical access networks [3, 4]. Since a DDO- OFDM system directly detects the signal using a square law photodetector, it must have a spectral gap between the optical carrier and OFDM signal band to accommodate subcarrier-to- subcarrier beat products, which otherwise would contaminate the actual data. At optimum operating conditions, this required spectral gap needs to be either equal to or greater than the OFDM signal bandwidth [4]. Therefore, in a DDO- OFDM system, at least half of the signal bandwidth remains unused, reducing the effective optical spectral efficiency enormously. To utilize the unused spectral gap of a DDO- OFDM system a frequency interleaving method has been proposed, where two neighbouring DDO-OFDM channels are overlapped in such a way that the mandatory spectral gap of a channel is being occupied by the OFDM signal band of its neighbour and thereby enhancing the effective spectral efficiency significantly [5, 6]. In this paper, we demonstrate such a frequency interleaved DDO-OFDM system both by theoretical analysis and numerical simulations. Our results show that, frequency interleaving of two adjacent DDO- OFDM systems each carrying 24 Gb/s pseudo-random-bit- sequence (PRBS) data with a OFDM signal band of 10 GHz (actual signal bandwidth is 25 GHz including the spectral gap), can increase the spectral efficiency up to 50% over a standard DDO-OFDM system. II. SYSTEM DESCRIPTION In this section we at first describe the effective spectral efficiency the proposed system followed by the theoretical analysis of such a system. A. Effective Spectral Efficiency of Interleaved Optical OFDM System As shown in Fig. 1, a conventional DDO-OFDM channel modulated in optical single sideband with carrier (OSSB+C) format with an effective bandwidth of B can be redesigned by frequency interleaving of two adjacent OSSB+C formatted DDO-OFDM channels where upper sideband (Band-1) of one channel falls within the mandatory spectral gap between the lower sideband (Band-2) and optical carrier of other channel and vice versa. In order to facilitate demultiplexing at the receiver, frequency interleaved channels need to have an OFDM signal bandwidth of kB rather than ½ B , where ¼ < k < ½. Here k should not be ¼ or less, because otherwise the effective data bandwidth of two interleaved channels will be ½ B or less that is undesirable. Data band of each channel is then surrounded by a bandwidth gap equals to ( ) /2 ½ B kB - , and the mandatory spectral gap of each channel becomes / 2) (½ ¼ - k B + , which is larger than ½ B compared to conventional systems. However, because of using two OFDM signal bands, the effective spectral efficiency of such a system increases significantly, which can be up to 50% (for k = 3/8) compared to conventional DDO-