Dual Architecture Uplink Demonstration of a 7×622 Mbps SAC-OCDMA PON Using a Burst-Mode Receiver

Ziad A. El-Sahn, Ming Zeng, Bhavin J. Shastri, Noha Kheder, David V. Plant, Leslie A. Rusch
2008 OFC/NFOEC 2008 - 2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference  
We demonstrate experimentally the uplink of an incoherent spectral amplitude-coded optical code-division multiple-access passive optical network (PON) using a burst-mode receiver. Error free transmission is achieved for local sources and centralized sources PON architectures. Introduction Spectral amplitude-coded (SAC) optical code-division multiple-access (OCDMA) offers cost savings over other OCDMA techniques as it requires electronics operating at only the bit rate and it enjoys excellent
more » ... enjoys excellent multiple access interference (MAI) rejection due to balanced detection [1] . Furthermore, advances in writing fiber Bragg gratings (FBGs) make possible the design of low cost and compact passive encoders/decoders well adapted to passive optical networks (PONs). OCDMA combines the large bandwidth of the fiber medium with the flexibility of the CDMA to achieve high-speed connectivity. SAC-OCDMA is suitable for PON applications, where cost and performance are critical. In this paper we examine for the first time to our knowledge SAC-OCDMA with local sources (LS) and centralized light source (CLS) PON architectures. An inexpensive incoherent light source is placed at each optical network unit (ONU) for LS architectures; a single high power light source is placed at the optical line terminal (OLT) for the CLS architectures. Most research in OCDMA focuses on optical design, and assumes the availability of compatible electronics [1], [2] . Emerging research is concerned with the electronic design of receivers for optical access networks, featuring post-processing functionalities [3] . Previous electronic receivers were reported in the literature for fast-frequency hop (FFH) OCDMA and PON systems [4], [5] . FFH-OCDMA requires electronics that operate at the chip rate rather than the bit rate. In this paper we demonstrate experimentally burst-mode reception of an incoherent SAC-OCDMA PON uplink supporting seven asynchronous users at 622 Mbps (FFH results were at 155 Mbps data rate) with no global clock, using a standalone receiver with a commercial SONET clock-and-data recovery (CDR). The receiver also features clock-and-phase alignment (CPA), forward-error correction (FEC), and a custom bit error rate tester (BERT), all implemented on a field programmable gate array (FPGA). We measure the performance of the proposed PONs in terms of bit error rate (BER) and packet loss ratio (PLR). We quantify the increase in soft capacity via FEC, while working with a nonideal recovered clock that provides realistic, achievable sampling. SAC-OCDMA PON physical architectures and burst-mode receiver functionalities Tree architectures are widely deployed for PONs, and both two-feeder and single-feeder versions have been studied [6] . In two-feeder architectures, uplink and downlink traffic are sent on separate feeders; a single-feeder architecture carries both uplink and downlink on one fiber feeder. Fig. 1(a) shows our proposed SAC-OCDMA PON architectures. Both LS architectures (a source at each ONU) and CLS architectures (one source at the OLT) can exist with two-feeder and single-feeder topologies (shown within the optical distribution network). In the LS architecture an inexpensive light emitting diode (LED) is directly or externally modulated and located at each ONU. A CLS architecture places a single powerful light source at the OLT, with remote external modulation of that source at each ONU. In CLS PONs, coarse wavelength-division multiplexed (WDM) filters are needed at the OLT and ONU, as shown in Fig. 1(a) , to separate the continuous wave light for the uplink from the modulated downlink. The remote node (RN) consists of passive combiners and splitters, as in existing TDM PONs, so there is no need to upgrade the PON infrastructure; for WDM PONs the couplers must be replaced by arrayed waveguide gratings (AWGs). At the OLT a multi-user transceiver is used to communicate with all users, optical amplifiers may be used to boost the signal prior to transmission and/or reception. Recall that the optimum receiver for a SAC-OCDMA PON at the OLT and ONUs is the conventional balanced receiver, where a 1×2 coupler, a decoder (DEC), a complementary decoder, and a balanced photodiode are used. Although our OCDMA PON architectures offer the flexibility of adopting both two-feeder and single-feeder architectures, we test only the two-feeder architecture. The effect of Rayleigh backscattering, which is reduced in two-feeder architectures, is not addressed. a1484_1.pdf OMR3.pdf OFC/NFOEC 2008 978-1-55752-855-1/08/$25.00 ©2008 IEEE Log (BER) Worst case Log(PLR) Fig. 2. (a) BER vs. power for LS and CLS PON architectures for different number of users, and (b) worst case PLR (at π phase shift) vs. number of users for LS and CLS architectures, with and without CPA. a1484_1.pdf OMR3.pdf OFC/NFOEC 2008
doi:10.1109/ofc.2008.4528393 fatcat:dtn6cdrajjeelof63p7qwsm5em