Fiber-Wireless (FiWi) Access Networks: Challenges and Opportunities

MAIER LAYOUT 1/5/11 12:09 PM Page 36 Fiber-Wireless (FiWi) Access Networks: Challenges and Opportunities Navid Ghazisaidi and Martin Maier, Optica...
Author: Nathan Walker
0 downloads 2 Views 238KB Size
MAIER LAYOUT

1/5/11

12:09 PM

Page 36

Fiber-Wireless (FiWi) Access Networks: Challenges and Opportunities Navid Ghazisaidi and Martin Maier, Optical Zeitgeist Laboratory, INRS Abstract Hybrid fiber-wireless (FiWi) access networks aim at combining the huge amount of available bandwidth of optical networks and the ubiquity and mobility of wireless access networks with the objective to reduce their cost and complexity. This article highlights key enabling optical as well as wireless technologies and explains their role in emerging FiWi networks. After briefly reviewing previous art, important challenges and imperatives for the design of future FiWi network architectures, protocols, and algorithms are identified and discussed in detail.

O

ptical fiber provides an unprecedented bandwidth potential that is far in excess of any other known transmission medium and offers significantly longer ranges without requiring any active devices. Optical fiber has some further advantageous properties such as longevity and low maintenance costs which will eventually render fiber the medium of choice in wired first/last mile access networks. In fact, this trend can already be observed in most of today’s greenfield deployments where fiber rather than copper cables are installed for broadband access [1]. Optical access networks provide transparency against data rate and signal format, which eased carriers worldwide into deploying future-proof passive optical network (PON) outside plants that can be flexibly upgraded as new technologies mature or new standards evolve [2]. In this article we elaborate on the final frontier of optical networks: the convergence with their wireless counterparts. Optical and wireless technologies can be thought of as quite complementary and will expectedly coexist over the next decades. Future broadband access networks will be bimodal, capitalizing on the respective strengths of both technologies and smartly merging them in order to realize future-proof fiber-wireless (FiWi) networks that strengthen our information society while avoiding its digital divide. By combining the capacity of optical fiber networks with the ubiquity and mobility of wireless networks, FiWi networks form a powerful platform for the support and creation of emerging as well as future unforeseen applications and services (e.g., telepresence) [3]. The remainder of this article is structured as follows. The next section introduces FiWi networks as a broad view of wireless and fiber optic network codesign and codevelopment instead of seeing them as two orthogonal technologies, and elaborates on the main differences to related topics such as optical wireless integration (OWI) and radio-over-fiber (RoF) networks. We then review the state of the art of FiWi networks. We outline challenges and imperatives of future FiWi networks, and highlight selected testbeds and lessons learned. The final section concludes the article.

36

0890-8044/11/$25.00 © 2011 IEEE

FiWi Networks: A New Research Area Due to the difficulty and prohibitive costs of supplying optical fiber to all end-user premises as well as the spectrum limitations of wireless access networks, hybrid FiWi access networks seem to be more attractive than relying on either standalone access solution. FiWi networks are realized by integrating wireless access technologies (e.g., cellular, WiMAX, and WiFi) with installed optical fiber infrastructure that has been pushed ever closer toward end users over the last few years.

Optical Wireless Integration Current copper-based access network technologies such as digital subscriber line (DSL) and hybrid fiber-coax (HFC) face serious challenges to meet the requirements of future broadband access networks. While DSL suffers from severe distance and noise limitations, HFC is insufficient to efficiently carry data traffic due to its upstream noise and crosstalk accumulation. Recent progress in optical fiber technologies, especially the maturity of integration and new packaging technologies, has rendered optical fiber access networks a promising low-cost broadband solution. In particular, PONs are able to provide lower network deployment and maintenance costs as well as longer distances than current DSL and HFC networks. Optical wireless integration (OWI) aims at integrating PONs and other optical fiber access technologies with emerging broadband wireless access technologies (e.g., WiMAX) in order to increase the capacity of wireless access networks and reduce access point complexity through centralized management [4].

Radio-over-Fiber vs. Radio-and-Fiber Networks RoF networks have been studied for many years as an approach to integrate optical fiber and wireless networks. In RoF networks, radio frequencies (RFs) are carried over optical fiber links between a central office (CO) and multiple low-cost remote antenna units (RAUs) in support of a variety of wireless applications, such as wireless local area networks (WLANs) [5] and microcellular radio systems [6]. It was

IEEE Network • January/February 2011

MAIER LAYOUT

1/5/11

Wireless access technology

12:09 PM

Page 37

Architecture

PON is used

WMN is used

Routing performed by

Channel assignment by

Level of QoS

Reconfiguration is provided

Protection is provided

Independent

Yes

No

BS

BS

Low

No

No

Hybrid

Yes

No

ONU-BS

ONU-BS

Medium

No

No

Unified connection-oriented

Yes

No

ONU-BS

ONU-BS

High

No

No

Microwave-over-fiber

Yes

No

Macro-BS

Macro-BS

High

Yes

No

Unidirectional ring

No

No

AP

AP

Medium

Yes

No

Bidirectional ring

No

No

CO

AP

Low

No

Yes

Hybrid star-ring

No

No

CO

AP

Medium

Yes

Yes

Unidirectional ring/PON

Yes

Yes

ONU-WG

ONU-WG

High

Yes

Yes

WiMAX

WiFi

BS: base station, AP: access point, WG: wireless gateway, ONU: optical network unit, OLT: optical line terminal, CO: central office

Table 1. Comparison of different FiWi networks. experimentally demonstrated that RoF networks can have an optical fiber range of up to 50 km. However, inserting an optical distribution system in wireless networks may have a major impact on the performance of medium access control (MAC) protocols [5]. The additional propagation delay may exceed certain timeouts of wireless MAC protocols, resulting in deteriorated network performance. More precisely, MAC protocols based on centralized polling and scheduling (e.g., IEEE 802.16 WiMAX) are less affected by increased propagation delays due to their ability to take longer latency between the CO and wireless subscriber stations (SSs) into account by means of interleaved polling and scheduling of upstream transmissions originating from different SSs. However, in distributed MAC protocols such as the widely deployed distributed coordination function (DCF) in IEEE 802.11a/b/g WLANs, the additional propagation delay between wireless stations (STAs) and access points (APs) poses severe challenges. Due to the acknowledgment (ACK) timeout, optical fiber can be deployed in WLAN-based RoF networks only up to a maximum length to ensure proper operation of DCF [3]. The aforementioned limitations of WLAN-based RoF networks can be avoided in so-called radio-and-fiber (R&F) networks [7]. While RoF networks use optical fiber as an analog transmission medium between a CO and one or more RAUs with the CO being in charge of controlling access to both optical and wireless media, in R&F networks access to the optical and wireless media is controlled separately by using in general two different MAC protocols in the optical and wireless media, with protocol translation taking place at their interface. As a consequence, wireless MAC frames do not have to travel along the optical fiber to be processed at the CO, but simply traverse their associated AP and remain in the WLAN, thus avoiding the negative impact of fiber propagation delay on network performance.

Fiber-Wireless Networks FiWi networks involve the deployment of both RoF and R&F technologies. R&F networks are well suited to build WLANbased FiWi networks of extended coverage without imposing stringent limits on the size of the optical backhaul, as opposed to RoF networks that limit the length of deployed fibers to a couple of kilometers. By simultaneously providing wired and wireless services over the same infrastructure, FiWi networks

IEEE Network • January/February 2011

are able to consolidate optical and wireless access networks that are usually run independent of each other, thus potentially leading to major cost savings. FiWi networking research deals with the OWI of emerging optical and wireless broadband access technologies such as the wireless mesh network (WMN). FiWi is a holistic approach that brings under one umbrella the development of PHY, data link, and routing (path selection) problems. More specifically, at the PHY layer, FiWi research inquires new methods of optical RF generation exploiting fiber nonlinearities and various modulation techniques. It also includes the study of different remodulation schemes for the design of colorless (i.e., wavelength-independent) RAUs. While significant progress has been made at the PHY layer of FiWi and in particular RoF transmission systems, FiWi networking research on layer 2 related issues has begun only very recently. Among others, FiWi layer 2 research includes the joint optimization of performance-enhancing MAC mechanisms separately used in the wireless and optical network segments (e.g., wireless frame aggregation and optical burst assembly, hybrid access control protocols, and integrated path selection algorithms, as well as advanced resilience techniques).

State-of-the-Art FiWi Networks In this section we review the state of the art in FiWi network architectures and discuss previously addressed challenges. Table 1 shows the FiWi network architecture alternatives and challenges recently surveyed in [8], which are briefly summarized in the following. Several time-division multiplexing (TDM) Ethernet PON (EPON)-WiMAX integration approaches have been proposed, ranging from independent and unified connection-oriented R&F to microwave-over-fiber RoF architectures. FiWi network architectures are classified based on their wireless access technologies: WiMAX or WiFi. While PONs can be widely found in FiWi networks, WMNs were used rarely so far. Different challenges were addressed such as routing and wireless channel assignment, which can be performed completely either in the wireless domain by the BS or AP, or by an optical network element (e.g., CO or optical line terminal [OLT]). The level of provided quality of service (QoS) largely depends on the performance of the implemented routing and resource management algorithms, including

37

MAIER LAYOUT

1/5/11

12:09 PM

Page 38

RF

λ2 WDM laser

Optical switches

λ3 λN

Mod 1 Mod 2 Mod 3

Mux

λ1

Mod N

Central office λN OADM RAU1

RAU2

RAU3

RAUN Railway

Figure 1. Moving cell-based RoF network architecture for train passengers.

bandwidth allocation and channel assignment algorithms with absolute or relative QoS assurances. Reconfiguration is another previously addressed challenging issue that involves resource management in the wireless and/or optical part. For instance, in the unidirectional ring/PON architecture of Table 1, highly loaded ONU-wireless gateways (ONU-WGs) may be assigned to a lightly loaded PON by tuning their optical transceivers to the wavelength assigned to the lightly loaded PON, resulting in a decrease of network congestion and packet latency. In Table 1 the bidirectional ring and hybrid starring FiWi networks deploy spare optical fiber for protection, while the unidirectional ring/PON architecture relies on WMN for survivability. In the following, we review recent RoF- and R&F-based FiWi network design proposals not covered in [8].

RoF Networks Moving Cell — Cellular networks used for fast moving users like train passengers suffer from frequent handovers when hopping from one BS to another one. The frequent handovers may cause numerous packet losses, resulting in a significantly decreased network throughput. An interesting approach to solve this problem is the use of an RoF network installed along the rail tracks in combination with the so-called moving cell concept [9]. Figure 1 depicts the moving cell-based RoF network architecture for train passengers. An optical wavelength-division multiplexing (WDM) ring interconnects the RAUs with the CO where all processing is performed. Each RAU deploys an optical add-drop multiplexer (OADM) fixed tuned to a separate wavelength channel. At the CO, a WDM laser generates the desired wavelengths, which are optically switched and passed to an array of RF modulators, one for each RAU. The modulated wavelengths are multiplexed onto the optical fiber ring and received by each addressed RAU on its assigned wavelength. An RAU retrieves the RF signal and transmits it to the antennas of a passing train. In the upstream direction, the RAU receives all RF signals and sends them to the CO for processing. By processing the received RF signals, the CO is able to keep track of the train location and identifying the RAU closest to the moving

38

train. It then assigns downstream RF signals to the corresponding RAU such that the train and moving cells move along in a synchronous fashion.

Moving Extended Cell — Very recently, the moving extended cell concept was proposed to provide connectivity in any possible direction [10]. The fiber optic network becomes a means for speedy handoff between BSs that serve mobile users. A hybrid frequency-division multiplexing (FDM)/WDM network architecture was used to support the delivery of multiple RF channels in the 60 GHz frequency band over the same wavelength. The extended cell involves the current user’s cell and the surrounding cells, ensuring connectivity in any random direction. The extended cell is adaptively restructured when the user enters a new cell. It was shown that the proposed concept can provide zero packet loss and call dropping probability in high-rate wireless services for a wide range of mobile speeds of up to 40 m/s independent of fiber link distances. SMF- vs. MMF-Based RoF Networks — RoF networks are attractive since they provide transparency against modulation techniques, and are able to support various digital formats and wireless standards in a cost-effective manner. While single-mode fibers (SMFs) are typically found in outdoor optical networks, many buildings have preinstalled multimode fiber (MMF) cables. Cost-effective MMF-based networks can be realized by deploying low-cost vertical cavity surface emitting lasers (VCSELs). Apart from realizing low-cost microcellular radio networks, optical fibers can also be used to support a wide variety of other radio signals. In [11], a low-cost MMF network was experimentally tested to demonstrate the feasibility of indoor radio-over-MMF networks for the in-building coverage of second-generation and third-generation cellular radio networks as well as IEEE 802.11b/g WLAN. Multiservice Access Networks — To realize future multiservice access networks, it is important to integrate RoF systems with existing optical access networks. In [12] a novel approach for simultaneous modulation and transmission of both RoF RF and fiber to the home (FTTH) baseband signals using a single

IEEE Network • January/February 2011

MAIER LAYOUT

1/5/11

12:09 PM

Page 39

RoF RF signal Wireless application external integrated modulator was experimentally demonstrated, as shown in Fig. 2. The external integrated modulator consists MZM 1 of three different Mach-Zehnder modulators Laser diode Optical filter (MZMs 1, 2, and 3). The RoF RF and FTTH MZM 3 SMF MZM 2 baseband signals independently modulate the optical carrier generated by a common FTTH laser diode by using MZM 1 and MZM 2, application respectively. Subsequently, the optical wireFTTH baseband signal less RF and wireline baseband signals are combined at MZM 3. After propagation Figure 2. Simultaneous modulation and transmission of FTTH baseband signal and over an SMF downlink, an optical filter (e.g., RoF RF signal using an external integrated modulator consisting of three Machfiber grating) is used to separate the two sigZehnder modulators (MZMs). nals and forward them to the wireless and FTTH application, respectively. It was experimentally demonstrated that a 1.25 Gb/s congestion control methods at the optical-wireless interface baseband signal and a 20 GHz 622 Mb/s RF signal can be node to maximize network throughput is an open issue. simultaneously modulated and transmitted over 50 km standard SMF with acceptable performance penalties. R&F Networks In [13] wireless intermediate frequency (IF) transmission over a WDM PON was experimentally investigated. The proR&F networks are those where the wireless and fiber optic posed network was designed to provide service for third-genpart are codesigned without, however, the fiber optic carrying eration cellular and WiMAX subscribers, as well as wired the native wireless signal (in any form). R&F networks exploit optical subscribers. A single wavelength for the downstream some similarities found in the tasks that both networks have direction and multiple wavelengths for the upstream direction to accomplish. For example the scheduling and the bandwidth were considered. The reported results show that a WDM allocation. Both EPON and WiMAX networks typically have PON with 8 orthogonal FDM (OFDM) channels is able to a point-to-multipoint topology with a central control station support 32 ONUs with a 3 dB power penalty. (OLT in EPON and BS in WiMAX) performing dynamic bandwidth allocation (DBA) by means of centralized polling Protocols and Algorithms and scheduling. These similarities give rise to interesting convergence problems whose optimization is expected to lead to Most RoF related work was focusing on the physical layer, an improved FiWi network performance. with a particular focus on downstream transmissions. As explained earlier, introducing extended fiber links in wireless networks may have a detrimental or even devastating impact Scheduling and Bandwidth Allocation — The simulation results on the throughput of distributed MAC protocols. In the folpresented in [14] demonstrate the improvement of network lowing, we describe the challenges and open issues of protothroughput and end-to-end delay for different QoS demands, cols and algorithms in future RoF and R&F-based FiWi using both centralized and distributed scheduling approaches. networks: The integrated QoS-aware dynamic bandwidth allocation •Integrated channel assignment and bandwidth allocation: (DBA) scheme proposed in [15] supports bandwidth fairness To improve bandwidth efficiency, powerful load balancing and at the ONU-BS interface, while the WiMAX SSs perform reconfiguration techniques may be used in FiWi networks, class-of-service bandwidth assignment. The reported results where both bandwidth (of the fiber medium) and spectrum show improvement of network throughput, delay, and band(of the wireless medium) must be dynamically allocated to width utilization. provide better service to hotspot BSs/APs. To give an example of the complexity of codesigning a •Integrated path selection: An open issue is the design of scheduling and bandwidth allocation scheme for fiber and logical topologies of reconfigurable optical backhaul networks wireless integration, Fig. 3 depicts the optical-wireless intersuch that the number of required handovers especially for face between a resilient packet ring (RPR) metro edge ring high-speed mobile customers can be decreased or avoided and a WiMAX access network, where the so-called integrated completely. rate controller (IRC) plays a key role in integrating the two •Optical burst assembly and wireless frame aggregation: networks [16]. It comprises a BS controller, a traffic class High-throughput next-generation 802.11n WLANs use two mapping unit, a central processing unit (CPU), and a traffic frame aggregation schemes known as aggregate MAC protoshaper. The IRC is used to seamlessly integrate both networks col data units (A-MPDU) and aggregate MAC service data and jointly optimize the RPR scheduler and WiMAX downunits (A-MSDU) which can be used separately or jointly to link (DL) and uplink (UL) schedulers. The BS controller is increase the MAC throughput. EPON does not support frame responsible for handling incoming and outgoing WiMAX trafaggregation. The benefit of one-level and/or two-level aggrefic, besides providing handover for SSs between different gation in EPON and converged EPON-WLAN networks is an interface nodes. The traffic class mapping unit is able to transopen issue. To provide QoS and bandwidth-efficiency of FiWi late the different WiMAX and RPR traffic classes bidirectionnetworks, the design of hierarchical optical burst assembly ally. The traffic shaper checks the control rates of RPR traffic and wireless frame aggregation schemes represents an exciting and performs traffic shaping according to RPR’s fairness poliavenue for future FiWi research. cies. The aforementioned units are synchronized and control •Flow and Congestion Control: The bandwidth disparity the RPR and WiMAX schedulers, and the incoming traffic between optical and wireless transmission media poses chalfrom each domain is monitored and influences the scheduling. lenges at the network level. Traffic transiting from the optical The proposed hierarchical RPR-WiMAX integrated schedulto the wireless network could exceed its capacity and congest ing algorithm significantly improves the throughput-delay perthe wireless network, leading to significant buffer overflows formance and provides triple-play QoS support (i.e., voice, and packet retransmissions. Developing appropriate flow and video, and data) for fixed and mobile WiMAX SSs.

IEEE Network • January/February 2011

39

MAIER LAYOUT

1/5/11

12:09 PM

Page 40

RPR optical fiber Transit in

Transit out In-transit traffic

PTQ

RPR scheduler

Checker STQ

Fair bandwidth allocator

Stage queue IRC Egress traffic

BS controller

Mapping

Traffic shaper

CPU

Ingress traffic UL scheduler ULrequest

DL scheduler

ULgrant

WiMAX antenna

Figure 3. Optical-wireless interface between RPR and WiMAX networks.

Routing Approaches — In [17] a novel integrated routing algorithm with load balancing was proposed for the unidirectional ring/PON architecture to improve the network throughput-delay performance. Moreover, different routing algorithms have been proposed for the wireless segm e nt o f F i W i n e t w o r k s [1 8 ]: mi n i mu m -ho p ro u ting algorithm (MHRA), shortest path routing algorithm (SPRA), predictive-throughput routing algorithm (PTRA), delay-aware routing algorithm (DARA), and risk-anddelay-aware routing algorithm (RADAR). Among the aforementioned routing algorithms, RADAR shows the best performance in terms of delay, throughput, and load balancing under both high and low traffic loads, besides providing risk awareness. In PON-based FiWi access networks, the network performance heavily depends on the placement of ONUs. Different heuristics (e.g., random and deterministic methods) were studied in [18] to find the optimal placement of ONUs in terms of time complexity and installation cost.

Enabling Technologies Different enabling technologies must be developed to realize low-cost FiWi networks. In the following we briefly summarize some of the key enabling RoF and R&F technologies.

RoF Technologies Optical RF Generation — To avoid the electronic bottleneck, the generation of RF signals is best done optically. Novel optical RF generation techniques need to be experimentally studied, for example, all-optical wavelength up-conversion by means of cross-absorption modulation (XAM) in an electroabsorption modulator (EAM) or four-wave mixing (FWM) in nonlinear dispersion-shifted fiber. According to [19], external intensity and phase modulation schemes are currently the most practical

40

solutions for all-optical RF generation due to their low cost, simplicity, and long-distance transmission performance.

Multiplexing — The use of different multiplexing techniques, such as WDM and subcarrier multiplexing (SCM), must be investigated for multi-user modulation. Toward this end, the joint distribution of ultra wideband (UWB) and WiMAX radio signals over PON using single- and orthogonal-polarization multiplexing is a promising solution [20]. In this approach both UWB and WiMAX signals are modulated by an external OFDM modulator and are then jointly transmitted by means of polarization multiplexing. The reported results show the feasibility of polarization multiplexing over a 25 km SMF PON with tolerable error vector magnitude (EVM) penalties. Remote Modulation — To build low-cost FiWi networks, different remote modulation techniques need to be studied. With remote modulation, a central light source is used at the CO to generate a downlink wavelength that is reused at RAUs for upstream transmission by means of remote modulation, thereby avoiding the need for an additional light source at each RAU. A detailed study of novel remote modulation schemes is required. An interesting low-cost and easy to maintain solution is the use of phase modulation for downstream and directly modulated colorless (i.e., wavelength-independent) semiconductor amplifier (SOA) for upstream transmission [19].

R&F Technologies Different optical technologies are expected to play an increasingly important role in the design of a flexible and cost-effective optical backhaul for FiWi networks, including tunable transceivers, burst-mode laser drivers and receivers, and colorless ONUs [21]. Among others, experimental studies are needed to investigate the dynamic sensitivity recovery as well as fast level and clock recovery of burst-mode receivers.

IEEE Network • January/February 2011

MAIER LAYOUT

1/5/11

12:09 PM

Aware home residential (receiver) Filter 2.4GHz O/E BS

Page 41

O/E

Sony Blu-Ray Centergy disk player building (transmitter) HD video 60GHz all-optical A/D up-conversion

60GHz BS

2.4GHz 60GHz 2.4GHz DownDownDownconversion conversion conversion D/A

E/O

Georgia Tech campus fiber network

D/A

Canon camcorder

Laptop SDTV

2.4GHz local oscillator (LO)

Optical combiner

A/D SD video

HDTV

Figure 4. Georgia Institute of Technology RoF field demonstration of SD/HD video delivery using 2.4 GHz and 60 GHz millimeter-wave transmissions [22].

Router Router

Gateway 1 ONU 1 Video server

Data server

Router

OLT 1

Router

1:8 Splitter

Gateway 2

OLT 2

VoIP client

Router

ONU 2

UC Davis network

VoIP client

Data client

Router

1:8 Splitter ONU 3

CO

Video client

Router Gateway 3

Router Optical backhaul Wireless front-end

Figure 5. UC Davis R&F testbed integration of EPON and WMN for voice, video, and data traffic [23].

Example Recent Testbeds

R&F Testbed

RoF Testbed

Figure 5 shows the University of California (UC) Davis R&F testbed integration of two EPONs and an IEEE 802.11g WLAN-based WMN with a maximum transmission rate of 54 Mb/s for voice, video, and data traffic [23]. In this architecture optical protection is provided by using full PON duplication. Programmability was realized by using a separate Linux PC connected to each ONU, and open source firmware in each wireless gateway and router. The results show that the quality of video transmissions sharply deteriorates for an increasing number of wireless hops. In fact, the video client showed a blank screen after four wireless hops. The experimental results clearly show that running EPON and WMN networks independently yields poor FiWi network performance. A more involved experimental investigation of integrated FiWi network architectures is needed taking hybrid access control protocols, integrated path selection algorithms, and advanced resilience techniques into account.

Figure 4 shows the RoF testbed designed at the Georgia Institute of Technology for the field trial demonstration of 270 Mb/s standard definition (SD) and 1.485 Gb/s high definition (HD) real-time video stream delivery using 2.4 and 60 GHz millimeter-wave transmissions over 2.5 km SMF between the Centergy building (transmitter) and the aware home residential building (receiver) [22]. All-optical upconversion is used at the transmitter to generate a 60 GHz millimeter-wave signal (by means of phase modulation) and to send the HD video signal at 1554 nm. As shown in Fig. 4, electrical mixing and double-sideband optical modulation techniques are used to upconvert the SD video 2.4 GHz radio signal before optical transmission at 1550 nm. PIN photodiodes are used at the receiver to perform O/E conversion of the filtered optical signals. The experimental results demonstrate a very good bit error rate (BER) performance of the received video signals.

IEEE Network • January/February 2011

41

MAIER LAYOUT

1/5/11

12:09 PM

Page 42

Conclusions The few testbeds in existence or under development present a fraction of the possibilities existing in the integrated design of fiber and optical networks, which we call FiWi networks. By seamlessly converging optical and wireless access technologies, hybrid FiWi access networks hold great promise to support a plethora of future and emerging broadband services and applications on the same infrastructure. In this article we briefly summarize the state of the art of FiWi network architectures and previously addressed challenges. We identified challenges and opportunities for the design of future FiWi networks. Many open issues related to the design of low-cost physical layer components and layer 2 protocols and algorithms must be solved in order to render FiWi access solutions commercially viable. We highlight two recent RoF and R&F testbeds, and outline future challenges and opportunities related to hybrid MAC protocols, integrated path selection, integrated channel assignment and bandwidth allocation, optical burst assembly, and wireless frame aggregation, as well as flow and congestion control.

References [1] W. Fischer, “Point-to-Point FTTx,” in Broadband Access Networks: Technologies and Deployments, A. Shami, M. Maier, and C. Assi (Eds.), Springer, June 2009. [2] F. Effenberger et al., “An Introduction to PON Technologies,” IEEE Commun. Mag., vol. 45, no. 3, Mar. 2007, pp. S17–S25. [3] M. Maier, N. Ghazisaidi, and M. Reisslein, “The Audacity of Fiber-Wireless (FiWi) Networks (Invited Paper),” Proc. ICST ACCESSNETS, Las Vegas, NV, Oct. 2008. [4] Y. Luo et al., “Integrating Optical and Wireless Services in the Access Network,” Proc. OFC/NFOEC, Anaheim, CA, Mar. 2006. [5] B. L. Dang and I. Niemegeers, “Analysis of IEEE 802.11 in Radio over Fiber Home Networks,” Proc. IEEE Conf. Local Comp. Net ., Nov. 2005, pp. 744–47. [6] T.-S. Chu and M. J. Gans, “Fiber Optic Microcellular Radio,” IEEE Trans. Vehic. Tech., vol. 40, no. 3, Aug. 1991, pp. 599–606. [7] P. S. Henry, “Integrated Optical/Wireless Alternatives for the Metropolitan Environment,” IEEE Commun. Society Webinar, Apr. 2007. [8] N. Ghazisaidi, M. Maier, and C. M. Assi, “Fiber-Wireless (FiWi) Access Networks: A Survey,” IEEE Commun. Mag., vol. 47, no. 2, Feb. 2009, pp. 160–67. [9] B. Lannoo et al ., “Radio-over-Fiber-Based Solution to Provide Broadband Internet Access to Train Passengers,” IEEE Commun. Mag., vol. 45, no. 2, Feb. 2007, pp. 56–62. [10] N. Pleros et al ., “A 60 GHz Radio-Over-Fiber Network Architecture for Seamless Communication With High Mobility,” IEEE/OSA J. Lightwave Tech., vol. 27, no. 12, June 2009, pp. 1957–67. [11] A. Das et al., “Design of Low-Cost Multimode Fiber-Fed Indoor Wireless Networks,” IEEE Trans. Microwave Theory Techniques, vol. 54, no. 8, Aug. 2006, pp. 3426–32. [12] C.-T. Lin et al., “Hybrid Optical Access Network Integrating Fiber-to-theHome and Radio-Over-Fiber Systems,” IEEE Photonics Tech. Letters, vol. 19, no. 8, Apr. 2007, pp. 610–12. [13] J. Hu, D. Qian, and T. Wang, “Wireless Intermediate Frequency Signal over Passive Optical Networks: Architecture and Experimental Performance Evaluation,” Proc. OFC/NFOEC, San Diego, CA, Feb. 2008, pp.1–6. [14] Y. Luo et al., “QoS-Aware Scheduling over Hybrid Optical Wireless Networks,” Proc. OFC/NFOEC, Anaheim, CA, Mar. 2007, pp. 1–7. [15] K. Yang et al., “Convergence of Ethernet PON and IEEE 802.16 Broadband Access Networks and its QoS-Aware Dynamic Bandwidth Allocation Scheme,” IEEE JSAC, vol. 27, no. 2, Feb. 2009, pp. 101–16.

42

[16] N. Ghazisaidi, F. Paolucci, and M. Maier, “SuperMAN: Optical-Wireless Integration of RPR and WiMAX,” OSA J. Optical Net., vol. 8, no. 3, Mar. 2009, pp. 249–71. [17] W.-T. Shaw et al., “Reconfigurable Optical Backhaul and Integrated Routing Algorithm for Load Balancing in Hybrid Optical-Wireless Access Networks,” Proc. IEEE ICC, Beijing, China, May 2008, pp. 5697–5701. [18] S. Sarkar, S. Dixit, and B. Mukherjee, “Hybrid Wireless-Optical Broadband-Access Network (WOBAN): A Review of Relevant Challenges,” IEEE/OSA J. Lightwave Tech., special issue on Convergence of Optical and Wireless Access Networks, vol. 25, no. 11, Nov. 2007, pp. 3329–40. [19] Z. Jia et al., “Key Enabling Technologies for Optical-Wireless Networks: Optical Millimeter-Wave Generation, Wavelength Reuse, and Architecture,” IEEE/OSA J. Lightwave Tech., special issue on Convergence of Optical and Wireless Access Networks, vol. 25, no. 11, Nov. 2007, pp. 3452–71. [20] J. Perez et al ., “Joint Distribution of Polarization-Multiplexed UWB and WiMAX Radio in PON,” IEEE/OSA J. Lightwave Tech., vol. 27, no. 12, June 2009, pp. 1912–19. [21] L. G. Kazovsky et al ., “Next-Generation Optical Access Networks,” IEEE/OSA J. Lightwave Tech., special issue on Convergence of Optical and Wireless Access Networks, vol. 25, no. 11, Nov. 2007, pp. 3428–42. [22] A. Chowdhury et al., “Advanced System Technologies and Field Demonstration for In-Building Optical-Wireless Network With Integrated Broadband Services,” IEEE/OSA J. Lightwave Tech ., vol. 27, no. 12, June 2009, pp. 1920–27. [23] P. Chowdhury et al., “Hybrid Wireless-Optical Broadband Access Network (WOBAN): Prototype Development and Research Challenges,” IEEE Network, vol. 23, no. 3, May/June 2009, pp. 41–48.

Biographies N AVID G HAZISAIDI ([email protected]) is a research associate at the School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe. He received an M.Sc. degree in electrical engineering with specialization in telecommunications (with distinction) from the Blekinge Institute of Technology (BTH), Karlskrona, Sweden, in 2006. He received his Ph.D. degree in telecommunications from the University of Quebec, INRS, Montreal, Canada, in 2010. He was a researcher in the Computer Network (CN) group at the University of Basel, Switzerland, and participated in the prestigious European research project Biologically-Inspired Autonomic Networks and Services (BIONETS), September 2006 through January 2007. He was a visiting researcher at the Deutsche Telekom Laboratories (T-Labs), Berlin, Germany, and participated in the prestigious European research project ACCORDANCE (A Converged Copper-Optical-Radio OFDMA-Based Access Network with Gigh Capacity and Flexibility), January 2010 through April 2010. His research interests are in the area of MAC protocol design for FiWi networks. MARTIN MAIER [SM] ([email protected]) has been an associate professor with the Institut National de la Recherche Scientifique (INRS), Montreal, since May 2005. He received M.Sc. and Ph.D. degrees, both with distinction (summa cum laude) in electrical engineering from the Technical University of Berlin, Germany, in 1998 and 2003, respectively. He was a visiting researcher at the University of Southern California (USC), Los Angeles, in spring 1998 and Arizona State University (ASU), Tempe, in winter 2001. In summer 2003 he was a postdoc fellow at the Massachusetts Institute of Technology (MIT), Cambridge. Before joining INRS, he was a research associate at CTTC, Barcelona, Spain, November 2003 through March 2005. He was a visiting Pprofessor at Stanford University, California, October 2006 through March 2007. He was a recipient of the two-year Deutsche Telekom doctoral scholarship from June 1999 through May 2001. He is also a co-recipient of the 2009 IEEE Communications Society Best Tutorial Paper Award and the Best Paper Award presented at the International Society of Optical Engineers (SPIE) Photonics East 2000-Terabit Optical Networking Conference. He has served on the Technical Program Committees of IEEE INFOCOM, IEEE GLOBECOM, and IEEE ICC, and is an Editorial Board member of IEEE Communications Surveys & Tutorials as well as Elsevier’s Computer Communications. He has served as a reviewer of numerous major journals and conferences as well as book proposals and research grant applications.

IEEE Network • January/February 2011