Fi-Wi by keralaguest

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									Fiber-Wireless (Fi-Wi)
      Networks




                     Bharani Ronanki

                          Z1560870
Table of contents:
  1) Introduction
  2) Fi-Wi networks
  3) Technologies enabling Fi-Wi networks
  4) Architectures
  5) Challenges
  6) Summary
  7) References
Introduction:
Wireless and optical fiber networks have always been separate from each other .Wireless
networks on one hand were aimed at providing a highly bandwidth-constrained transmission
channel susceptible to a variety of impairments and allowing the flexibility to reach almost
everywhere. On the other hand, optical networks do not have the flexibility but provide a huge
amount of available bandwidth. With the ultimate goal of providing access to information when
needed ,wherever needed and whatever format it is needed, the vision of converging the optical
and the wireless networks is not only becoming a necessity but also plays a key role in future
communication networks. In today’s times, communication industry demands broadband access
technologies to provide a mixture of services such as voice, data, and video. On the flipside, the
services that are being provided by the industry are also increasing like video on demand,
multichannel HDTV, mobile TV, Online gaming etc.[3]

Currently, there are many radio access technologies available through different systems such as
GSM, CDMA, WLAN and WPAN. Initially, these systems were designed for specific services in
target operating environments. To accommodate the rapidly growing demand for multimedia
services and extend the internet success to mobile internet, future broadband wireless networks
are expected to provided data rates up to 100 Mb/s with wide-area coverage and up to 1 Gbps
with local area coverage, and to be flexible enough to accommodate a wide range of transmission
rates. Thus there is a great need for future networks to be bimodal, capitalizing the respective
strengths of both the optical and wireless networks. The capacity of the optical networks
combined with the ubiquity and mobility of wireless networks forms the basis for Fiber-Wireless
(Fi-Wi) Networks. [1][2][3]

Fi-Wi networks:
Currently available wireless services and standards such as the Wi-Fi, GSM are concentrated in
the lower microwave band which is the 2.4 GHz band. Emerging wireless standards such as the
WiMAX and 3GPP or long term evaluation (LTE) will further enhance existing wireless
transmission speeds and throughputs. However, they still operate within the lower microwave
regions (2-4 GHz).This poses the problem of congestion in the wireless spectrum. This problem
could be solved by using the unused bandwidths of the extremely high frequency microwaves in
the millimeter-wave frequency region for provision of future broadband services. The 60 GHz
unlicensed band very soon gained all the attention in this direction. But these high frequencies
also pose the problem of propagation loss characteristics of wireless signals at these frequencies,
pico or microcellular architectures are essential to provide efficient geographical coverage which
could only be achieved by a large deployment of antenna base stations. With rapid increase of
the throughput of each base station in such systems, the use of an optical fiber backbone is
required to provide broadband interconnection between the central office and all the antenna
base stations. This leads to the integration of optical and wireless broadband infrastructures via a
common background network that provides various other advantages apart from supporting both
wired and wireless connectivity. Figure 1 shows an overview of how a fiber-wireless network
would look like.[2]




                             Figure 1: Overview of a Fiber-wireless Network[7]




Technologies enabling Fi-Wi networks:
There has been quite a bit of research over Fi-Wi networks for the last two years and the
technologies that have been use to implement Fi-Wi are

      Free space optical ( FSO) , also known as optical wireless (OW)
      Radio over fiber (RoF)

Free space optical is a type of direct line-of-sight (LOS) optical communications that provides
point to point connections by modulating visible or infrared(IR) beams. It offers high band width
and reliable communication over the short distances. The transmission carrier is generated by
deploying either a high-power light emitting diode (LED) or a laser diode, while the receiver
may deploy a simple photo detector. Current FSO systems operate in full-duplex mode at a
transmission rate ranging from 100 Mbps to 2.5 Gbps, depending largely on weather conditions.
Given clear weather conditions, a clear line of sight between source and destination and enough
transmitter power, optical wireless networks can transmit up to several kilometers. Optical fiber
may be used at both source and destination to build high speed LANs, such as Gigabit Ethernet
net (GbE).

Radio over fiber allows analog optical link to transmit a radio frequency signal. There are
different techniques available to realize radio over fiber networks. Radio over fiber networks
provide both peer to peer and point to multi-point connections. Recently, a full-duplex radio over
fiber system providing 2.5 Gbps data transmission over 40 Km with less than 2 dB of power loss
was successfully tested.



Features                Free space optical                   Radio over fiber

Connectivity            Point-to-point                       Point-to point and point-to-multi-
                                                             point
Transmission mode       Full duplex                          Full duplex

Scalability             High in terms of bandwidth           Low in terms of bandwidth
                        Low in terms of user and service     High in terms of user and service
Availability            Low in fog                           High in fog
                        High in rain                         Low in rain
Interference            Background sunlight                  Electromagnetic signals

Spectrum License        Not required                         Required



                                  Comparison between FOS and RoF

Architectures:
 Millimeter-wave (mm-wave) fiber-wireless systems have the advantage of being able to exploit
the large unused bandwidth in the wireless spectrum as well as the inherent large bandwidth of
the optical fiber. Such hybrid architecture can potentially provide high data rates and throughput
with minimal time delay. The generic architecture of a mm-wave fiber-wireless architecture is
shown in Fig. 1. The conceptual infrastructure comprises a CO which is connected to a large
number of antenna BSs via an optical fiber network. Much research has been carried out on the
development and exploitation of optically-fed mm-wave wireless technologies with earlier work
focusing on fiber link configurations for wireless signal distributions .In general, there are three
possible methods to transport the mm-wave wireless signals over the optical link The choice of
the optical transport scheme will also determine the hardware requirements in the CO and
antenna BS.

Radio frequency over fiber
The simplest scheme for transporting mm-wave wireless signals via an optical fiber feed network
is to directly transport the mm-wave wireless signals over fiber (RF-over-fiber) without any need
for frequency translation at the remote BS. In this configuration, the mm-wave wireless signal is
externally modulated onto the optical carrier resulting in an optical double sideband (ODSB)
signal. The two sidebands are located at the wireless carrier frequency away from the optical
carrier. Upon detection at the BS, the mm-wave wireless signal can be recovered via direct
detection using a high-speed photo detector. RF-over fiber transport has the advantage of
realizing simple base-station designs with additional benefits of centralized control,
independence of the air-interface and also enabling multiwireless band operation. However one
of its major drawbacks is the requirement for high-speed optical modulation techniques that have
the ability to generate mm-wave modulated optical signals and also high-speed photo detection
schemes that directly convert the modulated optical signals back to mm-wave signals in the RF
domain. Another key issue is the significant effect of fiber chromatic dispersion on the detected
mm-wave wireless signals.[1][2][3]

Intermediate Frequency over fiber

In contrast to the transmission of mm-wave wireless signals over fiber, the wireless signals can
be down converted to a lower intermediate frequency (IF) at the CO before optical transmission.
The effects of fiber chromatic dispersion on the optical distribution of IF signals are reduced
significantly. In addition, IF-over-fiber transport scheme has the advantage of using low speed
optoelectronic devices. The complexity of the antenna BS hardware however, increases with IF
signal transport for mm-wave wireless access systems. It will now require a stable mm-wave
local oscillator (LO) and high-speed mixers for the frequency translation processes in the BS.
This may also present a limitation when considering the ability to upgrade or reconfigure the
wireless network for the inclusion of additional mm-wave wireless channels or alterations to the
wireless frequency. The subsequent requirement for a mm-wave LO at the antenna BS can be
overcome by remotely delivering the LO signal optically from the CO.This also enables
centralized control of the LO signals themselves.[1][2]

Baseband over fiber

The third transport scheme transports the wireless signal as a baseband signal over fiber, and
then upconverts the information to the required mm-wave radio frequency at the antenna BS.
This scheme has the advantage of using mature digital and electronic circuitry for signal
processing at the BS. In addition, it also enables low-speed optoelectronic devices to be used
within the BS. As with IF-over-fiber, the effects of fiber chromatic dispersion are also greatly
reduced. Furthermore remote delivery of a mm-wave LO signal from the CO can overcome the
need of a physical LO in the BS. This transport scheme is dependent on the air-interface which
means that the BS must have the intelligence to thoroughly process the wireless signals before
sending the baseband information back to the CO. Hence, it necessitates the housing of
additional hardware within the BS to perform these tasks which increases the complexity of the
BS drastically. With the recent advancements in CMOS technology, high-frequency radio-on-
chip has been demonstrated.Also, the emergence of silicon photonic technology may enable the
future low-cost integration of optoelectronic and electronic devices in order to achieve a cost-
effective, compact transceiver module within the antenna BS.[2]

Discussion

Comparing the three transport schemes, ultimately there is a trade-off between the complexity in
the RF electronic and the optoelectronic interfaces within the base-station. One of the key
challenges in implementing mm-wave fiber-wireless access systems is to efficiently distribute
the wireless signals while maintaining a functionally simple and compact BS design. Amongst
the schemes, RF-over-fiber transport scheme has the potential to simplify the BS design for mm-
wave fiber-wireless systems. Having said this, the signals are susceptible to a number of
impairments along the link that may degrade the overall system performance. This will be
discussed in the next section.[2][3]

Challenges:
We have seen that Fi-Wi networks can be realized by deploying different architectures and
several technologies. Toward commercial adoption, Fi-Wi access networks still face a number of
technological challenges. One of the most critical challenges is to determine a feasible, scalable,
and resilient architecture along with the corresponding enabling technologies. As discussed
earlier, future broadband access networks will undoubtedly be a combination of first/last mile
optical fiber access solutions (i.e., FTTX) and heterogeneous broadband wireless networks
providing connectivity to end users. One first challenge is to seamlessly integrate these
technologies; while FTTX networks provide TDMA to wired ONUs, mobile client nodes in a
WMN access the medium through enhanced distributed channel access (EDCA) and multihop
routing used to forward their packets to wireless mesh gateways. New approaches to exploit the
huge bandwidth available in optical access networks for offloading bandwidth-limited wireless
networks should be studied in greater detail. The design and evaluation of powerful load
balancing and reconfiguration techniques to improve the bandwidth efficiency of future Fi-Wi
networks is another interesting research avenue, including reconfiguration techniques for
unpredictable traffic. Routing in WMNs remains a critical issue, and designing efficient routing
protocols that are aware of the bandwidth allocation on PON is more challenging; routing
algorithms that exploit this large bandwidth potential to offer fair access to WMN nodes as well
as load balancing across the mesh links are key for future Fi-Wi networks. Additionally, these
current access networks are designed to carry traffic with various QoS requirements. Various
QoS bandwidth allocation algorithms for PONs have emerged; however, designing QoS-aware
routing protocols in WMNs is still an open issue and is not addressed within the 802.11s
standard. In general, applications have different QoS requirements. Research on powerful end-to-
end resource allocation techniques in Fi-Wi networks is necessary. Resiliency against failures is
another challenge of future Fi-Wi networks. Fi-Wi networks should allow WMN gateways to
interconnect with the optical backhaul through multiple points in order to enable multipath
routing and improve their survivability. Additionally, the optical backhaul should implement
appropriate protection switching functions to deal with network element failures rapidly.
The 802.11s standard currently defines a new frame format for transmitting traffic over the
WMN. However, most of today’s deployed PON systems are based on EPON or BPON/GPON.
Therefore, interfaces are needed to allow for protocol adaptation and enable network
interoperability. Finally, implementation simplicity will be key to the commercial success of Fi-
Wi networks. Reducing the installation and protection costs by means of transferring expensive
devices and complex functions to the central office appears to be a promising approach to
building cost-effective Fi-Wi networks. In particular, cost-efficient and feasible modulation
formats for optical/RF signal conversion is needed. Despite recent developments in RoF
networks, more research on physical layer related issues is necessary due to the high atmospheric
absorption in high-frequency bands such as the millimeter wave band.[1][4][5]

Summary:

The special and unique characteristics of FiWi broadband networks attract us to design a novel
FiWi architecture with ability of providing end-to-end QoS connectivity for both wireless and
optical subscribers. Some of the characteristics of FiWi access networks are as follows:
Architecture

         Reduces costs and increases flexibility

Reconfigurability

         Allows for demand change among districts

         Improves network performance in terms of throughput, connectivity, and QoS

         One solution: Wavelength Division Multiplexing PON (WDM PON)

Load balancing

         Renders FiWi networks robust

Routing

        Plays a key role in wireless mesh networks which render robust wireless segment in
       FiWi access networks

Cost-efficiency and Migration

         Future-proofness by providing cautious pay-as-you-grow migration

        Backward compatibility with implemented standards as well as interoperability with
       future technologies

User-friendliness

       Autonomic FiWi networks: Self-configuring, Self-protecting, Self-optimizing, and
       Self-healing
References:
  1) J.J. O’Reilly, P.M.Lane, R. Heidemann and R.Hofstetter. “Optical generation of very narrow linewidth
     millimeter wave signals,” Electronic Letter. Vol 28, no.5, pp205-2311,1192.
  2) Christina Lim, Ampalavanapillai Nirmalaths,Masuduzzaman Bakaul, Prasanna Gamage, Ka-Lun Lee,
     Yizhuo Yang,Dalma Novak, Rod Waterhouse, “Fiber wireless networks and subsystem topologies,”
     Journal of Lightwave technology, Vol 28, no. 4, pp 390-403.
  3) Sonia Alissa and Martin Maier, “Towards Seamless Fiber-wireless access networks: Convergence and
     Challenges”, ICTON- MW 07
  4) Navid Ghazisaidi and Martin Maier, “ Fiber wireless(FiWi) Networks: A comparative Techno Economic
     Analysis of EPON and Wimax , Proc., Communications Workshop, Concordia University, Montréal, QC,
     Canada, Nov. 2009
  5) Navid Ghazisaidi and Martin Maier, “Fiber wireless (FiWi) Access Networks: A survey”, IEEE
     communications magazine, Feb 2009 , pp 160-167
  6) Zeyu Zhang, Jianping Wang, Jin Wang , “ A Study of Netwrok Throughput gain in optical wireless (FiWi)
     Networks Subject to Peer-to-Peer Communications”, IEEE ICC 2009 proceedings.
  7) Image, http://cdn.physorg.com/newman/gfx/news/hires/fiberwireless.jpg

								
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