Microsoft Word - ofc_2014_v6

To increase the number of subscribers in passive optical networks (PON), various multiple access schemes like time division multiple access (TDMA), frequency division multiple access (FDMA) and its special form orthogonal FDMA (OFDMA) can be used [1]. OFDMA is spectrally efficient and allows adaption to the channel and traffic demand by flexible bitand power-loading. The uplink (UL) direction in such an OFDMA-PON is the challenging part, since here the signals are generated and modulated at different locations and have to be multiplexed without distorting each other. For error free transmission these signals need to be orthogonal leading to requirements in time and frequency domain [2,3]. Most publications of FDMA-PON systems agree that the optical carrier must be generated centrally and distributed to the optical network units (ONUs) in order to ensure synchronization in frequency domain. However, depending on the receiver structure either synchronization of the uplink signals in time domain is required, which can be difficult to achieve and maintain [3,4]. Alternatively, sufficiently large band gaps are needed between the uplink signals which reduce the spectral efficiency [5,6]. In [2,7] we showed an OFDMAPON concept that allows closely packed subcarrier groups without the need of time synchronization between the uplink signals using individual FFTs per transmitting ONU. In this paper we experimentally compare the impact of asynchronous signal reception using this approach to standard digital signal processing (DSP) structures with a single FFT for all ONUs that require synchronous signals. The downlink transmission is expected to use a different wavelength and is not discussed here.