Effects of adaptive subcarrier modulation on MAC operation in OFDMA-PONs

We propose a MAC framework for OFDMA-PONs which allows exploiting both the subcarrier and time domains in a dynamic and flexible way. A dynamic subcarrier assignment algorithm that respects QoS by offering decreased average delay is proposed. Additionally, the proposed PON MAC framework for the first time leverages on a cross-layer optimization via the use of adaptive subcarrier modulation (ASM). ASM implies using a different modulation format per ONU depending on the transmission performance in individual links. The exact MAC procedures needed to support such a way of operation are discussed. Finally, delay performance of all schemes is evaluated using a hybrid PHY-MAC computer simulation tool. The simulation results on the one hand assist to determine the proper selection of MAC layer parameters (like the maximum number of subcarriers per ONU) and on the other demonstrate delay performance of the proposed scheduling scheme under realistic settings. Moreover, the employment of ASM is proven to provide significant delay reduction with acceptable physical layer performance degradation.

1. Introduction

Driven by rapidly increasing application bandwidth demands, standardization in the field of Passive Optical Networks (PONs) –one of the leading solutions providing Fiber-to-the-Home – has moved towards the concept of Next Generation PONs (NG-PONs). This implies both downstream and upstream aggregate rates of several tens of Gbps. Up to now in all PON solutions (like e.g. [1]) downstream/upstream capacity is shared among Optical Network Units (ONUs – located at the user premises) in the time domain. This is due to the use of a simple optical power splitter/coupler device at the Remote Node (RN) which allows significant reduction in fiber deployment costs. This implies that in the downstream direction traffic is broadcast to all ONUs, with each ONU keeping only the data destined to them each time, while in the upstream ONUs are allowed to transmit only within time slots dictated by the Optical Line Termination (OLT) Medium Access Control (MAC) layer (located at the Central Office–CO) in order to avoid collisions with transmissions from other ONUs (TDMA operation). However, it is constantly reported that lots of practical issues emerge when trying to scale up to higher capacities, mainly due to the need of very high-speed optical components and burst-mode TDM receivers [2]. Therefore, in line with NG-PON requirements, novel technological advances which will facilitate scaling up PON capacity and flexibility are needed.