Christian Ruprecht

37.5 km urban field trial of OFDMA-PON using colorless ONUs with dynamic bandwidth allocation and TCM [invited]

The orthogonal frequency division multiple access (OFDMA)-based passive optical network (PON) is a potential candidate to meet the flexibility requirements for next-generation optical access networks. We propose an OFDMA-based PON with a transmission employing intensity modulation/direct detection in the downstream and a remodulation of a remotely seeded carrier provided by the optical line terminal with coherent detection in the upstream, which enables cost-effective colorless optical network units (ONUs). Furthermore, an OFDMA-PON field trial using the proposed scheme over 37.5 km feeder fiber is demonstrated. A power budget supporting 32 ONUs with dynamic bandwidth allocation and trellis coded modulation (TCM) is reported.

Robust Transmission of 29-Gb/s OFDM Signal Over 1-km OM1 MMF Under Center Launching

Center launching is a simple and straightforward method to couple optical signals to a multimode fiber (MMF), and able to preferentially excite a few lower-order modes with reduced modal dispersion. However, system imperfections and mechanical perturbations will excite undesired higher order modes, leading to a selectively faded channel frequency response. For this reason, center launching is conventionally less favorable than offset launching when on-off keying is used. This problem can be circumvented by leveraging on the fine frequency granularity of the orthogonal frequency-division multiplexing (OFDM) signal, which is inherently less sensitive to the frequency fading. We have experimentally demonstrated the transmission of a 40-Gb/s OFDM signal (net data rate of 29.48 Gb/s) over 1-km G62.5/125 MMF without using complicated bit-and-power loading. The channel bandwidth is less than 9.1 GHz. We also demonstrate the robustness of the system against fiber bending, polarization variation, unmatched launch beam spot size, as well as its long-term stability.

Timing Advance Tracking for Coherent OFDMA-PON Upstream System

A synchronization algorithm for coherent OFDMA-PON upstream is proposed. The feasibility of the tracking functionality is experimentally investigated. A differential delay of up to 37.6 ns was tracked and could be compensated at the ONU.

Experimental IM/DD OFDMA transmission with scalable receiver frontend for PON scenarios

We propose a technique to reduce the electrical hardware effort in an OFDMA-PON by reducing the ADC and DSP complexity and therefore the costs of the ONUs and show its feasibility experimentally.

OFDM remodulation for 10-Gb/s/channel WDM-PON with simple carrier extraction and enhanced tolerance to Rayleigh noise

A novel OFDM-remodulation scheme for 10-Gb/s/channel WDM-PON is proposed and numerically investigated. Negative remodulation penalty is achieved without using any ultra-narrow filter or coherent detection. 40-km single-fiber bidirectional transmission is confirmed with 7-dB power margin.

Power budget improvement for coherent optical — OFDM access upstream transmission using TCM with constellation shaping

Power-budget improvement to support more Optical Network Units (ONU) is the most significant issue for passive optical access networks. Investigation on Trellis coded modulation for multi-user phase noise channel is conducted to improve the power budget for CO-OFDMA uplink transmission. The power budget is doubled enabling 37.4-km CO-OFDMA upstream transmission supporting 32 ONUs. To our best knowledge, this is the first field trial of optical access network using coherent optical - OFDM technique.

Initial ranging scheme based on interpolated Zadoff-Chu sequences for OFDMA-PON

Initial access to passive optical networks (PONs) requires upstream (US) synchronization of multiple optical network units (ONUs). We propose a low-complexity scheme for orthogonal frequency division multiple access (OFDMA) based PONs and demonstrate it experimentally. The scheme is based on interpolated Zadoff-Chu sequences, to synchronize the signal of each ONU arriving at the optical line terminal (OLT). Our experimental test bed consists of 4 colorless ONUs seeded by a single laser located at the OLT. Thermal expansion of a fiber is utilized to emulate a dynamic time drift in the demodulation-window positioning of OLT. The detected individual offsets at the OLT are fed back to the ONUs and are compensated at every data frame transmission. In this way, both inter carrier interference (ICI) and inter symbol interference (ISI) remain below acceptable levels.

OFDMA based PONs with reduced hardware requirements through advanced signal processing

Possibilities of reducing hardware requirements and therefore costs in an OFDMA-PON are shown. This is achieved by reducing the complexity of A/D and D/A converters and of the Digital Signal Processing (DSP) involved. The feasibility is shown experimentally as well. Furthermore, by incorporating techniques of the copper based access network, a flexible transmission network with advanced signal processing can be achieved and by applying techniques like bit and power loading, the data rate or the spectral efficiency can be increased. Moreover by assigning only certain subcarriers to various users, the effect of power fading can be mitigated.

Field trial of an OFDMA-PON system

OFDM is an interesting candidate to meet the requirements of high flexibility and speed for the next generation optical access networks. We show an OFDM system utilizing a timing advance tracking algorithm to realize a synchronized upstream transmission. Therefore, a dynamic bandwidth allocation can be used in the upstream and an interleaved assignment is made possible. We then demonstrate the feasibility in a field trial with four individual ONUs using a 37.5 km field deployed feeder fiber of the Deutsche Telekom AG. Power loading and the trellis coded modulation was used to achieve a power budget supporting 32 ONUs for up- as well as downstream transmission.