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Trends in Next Generation Optical Access Networks and a Proposed Hybrid Optical/Wireless Wide-area Access Network Junichi Kani NTT Access Network Service Systems Laboratories, NTT Corporation, Japan

Abstract— This paper first reviews trends toward the next-generation optical access networks; one of the key points is how to utilize wavelength-division multiplexing. It also overviews the technique of fiber-wireless access as a candidate to provide wireless connection in the last one step of the future access network. In the latter half, a novel hybrid optical/wireless access network is proposed; the importance of microwave photonics technologies is emphasized. The proposed network provides next-generation optical access and high radio frequency wireless access on the same platform.

1. INTRODUCTION

In the last few years, the optical access service based on Fiber-to-the-Home (FTTH) has been proven to be the most promising fixed Internet access service; the number of customers in Japan exceeded that of ADSL in early 2007. The typical Gigabit-capable passive optical network (PON) system shares 1 Gbit/s total bandwidth among 16 to 32 subscribers. A complementary technology to support broadband access is the wireless network using the microwave band; it forms the last one step of the access network, e.g., inside stations/floors. In these networks, average bandwidth per user is assumed to be around several tens of Mbit/s. This paper first reviews technologies and standardization trends toward the next-generation optical access networks (NG-OAN), whose target can be assumed to be providing new services with the average bandwidth of several hundreds of Mbit/s to several Gbit/s. One of the key points is how to utilize optical multiplexing technologies including wavelength-division multiplexing (WDM) in the NG-OAN [1]. It next overviews the technique of fiber-wireless access to provide the last one step of the future access network. Last, a novel hybrid dense-WDM and fiber-wireless access system is proposed for constructing the next-generation wide-area access network. In the network, microwave photonics play a key role in terms of multi-wavelength generation, as we reported before [2], as well as fiber-wireless transmission. The proposed system utilizes DWDM wavelengths divided to several wavelength groups, and each of the wavelength groups can provide high speed optical connection services (e.g., 10 Gbit/s) via full-fiber access and/or next-generation wireless services (e.g., 100 Mbit/s) via fiber-wireless access depending on the situation. 2. NEXT-GENERATION ACCESS NETWORK 2.1. Next-generation Optical Access

Gigabit-capable PON systems such as GE-PON (IEEE standard 802.3ah) and G-PON (ITU-T Recommendation G.984 series) are now being deployed to support broadband optical access services. These PONs use time-division multiple access (TDMA) to realize the point-to-multipoint connection between an optical line terminal (OLT) located in an operator’s building and the optical network units (ONUs) in the users’ homes. Each connection provides 1 Gbit/s (to 2.5 Gbit/s) total bandwidth that is shared among 16 to 32 subscribers; Figure 1(a) shows the typical schematic of the TDMA-based PON. The dynamic Bandwidth Allocation (DBA) algorithm is one of the keys to maximizing TDMA-PON performance while keeping fairness among subscribers. Gigabit capable PONs provide enough bandwidth for end users and they occupy the current sweet spot of opto-electronic components, i.e., maximum performance and minimum cost. However, considering the continuous evolution of communication services such as bidirectional video communication and three-dimensional image transmission, long-term efforts on the next generation system are important. As a candidate technology for NG-OAN, one attempt being extensively pursued is the WDMPON, where a dedicated bandwidth is provided by assigning a wavelength to each user. This scheme of assigning wavelengths instead of time slots (TDMA) is called wavelength-division multiple

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access (WDMA). Figure 1(b) shows a typical schematic of a WDM-PON with WDMA. A simple implementation of WDM-PON is to put a differently colored laser (i.e., a laser with a different wavelength) in each ONU, but then operators have to control who uses which wavelength as well as maintain stocks of devices for all wavelengths. To resolve such issues, technologies to realize “colorless ONUs” are being widely pursued [1]. One example is to employ wavelength-tunable lasers in ONUs and another is to remotely lock the wavelength of ONU lasers by sending the seed wavelength from the OLT side. A combination of TDMA and WDMA is also possible to increase the maximum bandwidth by WDM while retaining gradual bandwidth allocation made possible by TDMA. Dynamic Wavelength Allocation (DWA) algorithm has been researched for such networks [3]. 1 UNI

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Figure 1: Typical schematics of TDMA-PON and WDM-PON, (a) TDMA-PON, (b) WDM-PON.

Another possible approach to NG-OAN is to extend the reach so as to realize consolidation with metro networks [2, 4]. The motivation of these works is to reduce operational expenditure (OPEX) by reducing or eliminating elements that raise the cost of the operation, administration and maintenance functions in the operator’s buildings nearest to the end users. Extending PON reach and splitting ratio by the use of optical amplifiers has been extensively researched [4, 5]. The use of WDM is also important to increase the transmission capacity in the trunk line between two operator’s buildings [2]. As for the standardization trends of NG-OANs, IEEE has started discussion on 10GE-PON [6]. The 10GE-PON simply increases the speed of time-division multiplexing (TDM) to ten times the GE-PON level, where the issues are to realize opto-electronics that can work at 10-Giga speed as well as lower cost. The FSAN (full service access network) forum, which submitted B-PON and G-PON proposals to ITU-T, is also studying the next-generation access (NGA), where WDM functionality, reach extension, and evolution scenarios are under study in addition to increasing the speed of TDM [7]. 2.2. Fiber-wireless Access

Wireless access technologies are evolving to use multi-input/multi-output (MIMO) channels and/or higher radio frequencies for carrying the next generation services at over 100 Mbit/s. Among them, an interesting approach is the radio-on-fiber technology, which simplifies the system architecture by directly delivering high radio frequency broadband signals through optical fibers. One interesting approach is to heterodyne two optical frequencies whose difference corresponds to the radio frequency (RF), e.g., 60 GHz [8]. By modulating one of these two optical frequencies before fiber transmission, the RF signal can be extracted by heterodyne detection after fiber transmission. This approach can eliminate not only the local RF generator in each base station (BS), the boundary between the fiber link and the wireless link, but also provide better fiber-transmission performance than direct optical carrier modulation with a high RF signal. However, the optical phases of the two optical frequencies must be synchronized to generate a stable RF signal after detection. Multiwavelength light sources that can inherently provide such phase synchronization between any two wavelengths are an attractive candidate for this purpose as well as for multiplexing many different RF signals. DWDM (dense wavelength division multiplexing) extension of these radio-on-fiber access systems has been demonstrated; it multiplexes many sets of RF signals onto one feeder fiber to connect to the central station (CS); a supercontinuum-type multi-wavelength light source has been used [9].

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3. A PROPOSED HYBRID OPTICAL/WIRELESS WIDE-AREA ACCESS NETWORK

As described in the above section, the trends in next-generation optical access networks include reach extension for a wide-area access network that realizes consolidated operation to reduce OPEX while fiber-wireless access is interesting for simplifying the architecture of next-generation high speed wireless access. In this section, we propose a hybrid optical/wireless wide-area access network based on DWDM group multi/demultiplexing; Figure 2 illustrates its configuration. OLTs and an optical carrier supply module (OCSM) are located in the center node; the OCSM is a multi-wavelength light source (described later) that supplies multi-wavelength carriers to several OLTs. The OLT has interfaces for radio-on-fiber access (RoF IFs) as well as those for high speed optical access such as 10 Gigabit Ethernet (10GE IFs). This network uses DWDM wavelength channels (e.g., 64 channels) and the group multi/demultiplexer (G-MUX) in each remote node divides/combines the DWDM channels into/from several groups of wavelengths (e.g., 8 wavelengths ×8 groups). Each wavelength group is dedicated to a building/apartment, where a wavelength pair can provide either optical access via an ONU or RoF access with a BS (e.g., 1 × 10GE-ONU and 3×RoF BSs in each building/apartment). Center node Servers

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Figure 2: Proposed hybrid optical/wireless access network.

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Figures 3(a) and (b) show the configuration and optical spectrum of the OCSM that we reported previously [2]. By modulating seed laser diodes (LDs λ1 to λn in the figure) with a phase modulator (PM) and an intensity modulator (IM) using 25-GHz radio frequencies with appropriate amplitudes, we can obtain multi-wavelength optical carriers as the modulation sideband as shown in Figure 3(b); the spectrum consists of four wavelength groups where each group consists of eight wavelengths with 25-GHz spacing. Typically, low-cost optical filters for group multi/demultiplexing require an adequate guard-band between neighboring groups. As you can see in the spectrum, this OCSM is very suitable for such a group multi/demultiplexing because one can easily adjust the spacing between neighboring wavelength groups by adjusting the seed wavelengths. -20 -30 -40 -50 -60 -70 -80 1570

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Figure 3: OCSM, (a) configuration, (b) output optical spectrum.

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Figure 4 shows a detailed configuration of the proposed network system. Wavelength pairs to be used for an ONU or a BS are λk1 /λk3 , λk2 /λk4 , λk5 /λk7 , λk6 /λk8 where k is the group number (runs from 1 to n). In the case of the OCSM shown in Figure 3, n is 4; we can increase n by adding seed wavelengths to OCSM. The center node consists of n sets of modulation arrays (Mod array), n sets of receiver arrays (Rec array) and group multi/demultiplexers (G-MUX/G-DMX). Each Mod array multi/demultiplexes the eight wavelengths in each group by using arrayed-waveguide gratings (AWGs), and λk1 , λk2 , λk5 , λk6 are modulated by 10 Gbit/s binary NRZ data (for optical access) or 10 GHz-band IF data (for RoF access). In the remote node, the G-MUX/DMX divides/combines signals by wavelength groups. In the building, AWGs multi/demultiplex the eight wavelengths. BS receives the RoF signal by heterodyning the wavelength pair (e.g., λk1 /λk3 ) to generate a wireless signal with 60-GHz RF as well as transmitting the RoF signal by remotely modulating and transmitting one of the wavelength pairs (e.g., λk3 ) upstream [9]. ONU simply receives one modulated wavelength pair (e.g., λk2 ) as well as modulating/demodulating another wavelength pair (e.g., λk4 ). By using this system configuration, we can realize a wide-area access network that provides both high-speed optical access and fiber-wireless access flexibly depending on need. As described, microwave photonics technologies play a key role in terms of multi-wavelength generation as well as fiber-wireless transmission in the proposed network. 䎥䎶

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4. CONCLUSIONS

This paper first reviewed trends in the next-generation optical access networks; possible directions included how to utilize WDM technologies in the NG-OAN as well as reach extension to realize widearea access networks. Next, fiber-wireless technologies were described as an interesting candidate with which to deliver high radio frequency broadband signals through optical fibers directly. Last, a hybrid optical/wireless wide-area network was proposed for providing next-generation high-speed optical access and next-generation high radio frequency wireless access on the same platform. REFERENCES

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