Published on March 17th, 2011
At first glance, you might question the need to cover fiber-optic components in the wireless design space. But more and more often, fiber optics plays a role in the success of wireless systems. Bandwidth issues alone are a compelling reason for a fiber complement to wireless. So, it should not be surprising that many wireless-systems designers require a working knowledge of key fiber-optic components, such as photodetectors and laser diodes.
What exactly is the connection between fiber and wireless? As many communication companies have learned through painful experience, no one communication medium fits all needs. The best solution often turns out to be a multifaceted combination of traditional copper (Cu) cables, wireless links, and fiber optics.
This is especially true in light of the migration, albeit slow, to third-generation (3G) systems that require enormous bandwidths to carry both voice and data. Commercially available laser diodes and photodetectors now operate at rates above 10 Gb/s; the data equivalent of an analog 10 GHz.
What is the governing factor between wireless and fiber optics? Cost. Fiber-optic technology can be very expensive to install, much more so than creating a wireless infrastructure with a few antenna towers and base stations. Still, in addition to the advantage of tremendous bandwidth, fiber technology comes free of government regulated transmission channels, unlike its wireless counterpart. Also, since fiber-optic technology uses modulated light, as in the visible spectrum, there are no design issues with RF or electromagnetic interference (EMI).
What are the basics of designing with fiber-optic components? What vendors are providing fiber-optic components? And finally, what are the latest trends in this technology, as it pertains to the wireless community? These are issues we will now explore.
Reviewing a few of the basic elements of this technology will help give a clearer idea of the design issues inherent in optoelectronic systems. Fiber optics use light pulses to transmit information through fiber lines, in an analogous fashion to electronic pulses traveling down copper lines.
How do fiber-optic components work in conjunction with wire-based systems? Electrical signals must be converted to optical signals before optical data can be transmitted via fiber cables. The conversion occurs at the transmission end of the system, with the reverse - from optical to electrical signals - occurring at the receiving end. Transmission and receiver devices that contain both electrical and optical components handle these conversions.
1. This illustration, courtesy of Maxim, Inc., depicts a typical optical receiver/transmitter unit.
Photodiodes or photodetectors are the optical receivers that detect the transmission of optical signals from the fiber and convert them back into electrical signals. A relatively inexpensive component, the photodetector uses the same supply voltage as electrical components, e.g., +3.3 or +5 VDC.
Once converted the data must be amplified before it is ready for the clock and data-recovery (CDR) circuit, where the data waveform and clock are restored. Depending upon the bit rate and data type, a serial-to-parallel conversion of the data, via a demultiplexing unit, may then be necessary
Analog RF designers may be interested to note the importance of phase-locked-loop (PLL) functionality in the CDR circuit. The use of a PPL is necessary in synchronizing the clock with the data stream, to ensure proper alignment of the clock with the middle of a data work. This is just one aspect of the bit-error-rate (BER) reduction techniques used in optoelectronic systems.
The transmission process, e.g., going from the electrical to optical signal, is functionally the reverse of the receiving process. Data from electrical signals plus clock input are multiplexed and combined into one output stream. This is fed into a driver and then delivered to an optical source, such as the laser diode, for conversion to an optical signal.
Although fiber-optic transmissions do not have the same level of signal degradation and attenuation loss as are inherent in copper wire, they do lose integrity over distance. This is often described as the loss "windows, " which are graphically represented by the loss versus distance curve for fiber (Fig. 2).
For telecommunication networks, the two most important wavelength ranges are between 1000nm and 1300nm, called the second and third optical windows, respectively. The second optical window is known for low dispersion - as low as 0 dB. The third window provides the lowest attenuation per unit of fiber length.
2. This chart plots attenuation and dispersion versus wavelength.
Laser diodes have a high spectral purity that makes them well suited for long-distance and wavelength-division-multiplexing (WDM) transmission systems. New multiplexing techniques, such as WDM, are being used to increase transmission capacity by sending numerous time-multiplexed data streams over a single fiber. Each data stream has a different wavelength. Currently, the most widely used technique, time-division multiplexing (TDM), transmits a single data stream over each fiber cable.
Other modulation techniques are being refined; many having been "borrowed" from the RF world. One such technique is optical subcarrier multiplexing (SCM), which is essentially a form of frequency-division multiplexing (FDM). This approach will significantly increase a given portion of bandwidth in an optical system through channelizing.
The enormous bandwidths available from fiber-optic systems make them ideal for linking high-speed, wireless or wired local-area networks (LANs) and wide-area networks (WANs). Several high-speed applications use fiber as the physical-layer backbone, including:
• Fiber Distributed Data Interface (FDDI) - 100 Mb/s.
• Synchronous Optical Network (SONET) - 155 and 633 Mb/s.
• ESCON - 200 Mb/s.
• Gigabit Ethernet - 1000 Mb/s.
• Fibre Channel - 1062 Mb/s and below.
• High-Performance Parallel Interface (HIPPI) - 1200 Mb/s.
The basic requirements in designing an optical transmission system for telecommunications are not that different from wireless systems: power dissipation, supply voltage, integration level, and margin of performance. An additional constraint for fiber-optic designs is satisfying the recommendations of the International Telecommunication Union-Telecom Standards Sector (ITU-T). These recommendations address the quality of the optical signal by specifying limits on the tolerance, transfer, and generation of jitter.
Fiber-optic component vendors, such as Maxim, Inc. (Sunnyvale, CA), offer a variety of chip-set solutions to wireless designers that must interface with fiber. For example, the MAX3664 converts a single-ended current from the detector diode to a single-ended voltage, which is then amplified. Clock and data-recovery functions can be handled by the MAX3675 product. Wireless analog RF designers will note that this chip’s PPL, a necessary component for clock recovery, is integrated and does not require an external reference clock. The MAX3681 performs the deserialization (demultiplexing) of the signal into its constituent parts. This chip supports the various complementary-metal-oxide-semiconductor (CMOS) system-interface circuits that will interface with the rest of the wireless circuit.
Toshiba (New York, NY) offers a wide variety of optoelectronic components, including fiber-optic cable, photodetectors, laser diodes, and light-emitting diodes (LEDs). While inexpensive and suitable for short-distance local-area networks (LANs), LEDs have a broad spectral bandwidth that makes them unsuitable as optical transmitters for telecommunication systems.
There are many other quality vendors that offer high-performance fiber-optic components. The Optical Society of America (OSA) [www.osa.org] provides a wealth of information concerning fiber-optic conferences and component vendors.
Other companies, such as corporate giants like Alcatel (Plano, TX), Corning
Fiber (Corning, NY), Lucent Technologies (Murray Hill, NJ), and Nortel Networks (Mission Park, CA), provide both component as well as system-level solutions. Lucent, for example, is both a provider of WDM fiber-optic component hardware as well as fiber-optic networks. Lucent’s WaveStar Wireless Network Solution allows wireless operators to lease unused optical fiber lines or excess capacity on active fibers. Many wireless providers, faced with the bandwidth challenges of 3G, should find this capability of "acquiring " a fiber network particularly appealing.
Just as in the wireless world, the major trend facing fiber-optic component manufacturers is development of less expensive, small-sized modules. A key requirement for optoelectronic parts is to continually increase the transmitter and receiver data rates.
Perhaps one of the most innovative trends in optical-wireless connectivity is the through-air, optical network. Many environments, such as high-rise office buildings and one-time events, make it cost prohibitive to install an optical network. One possible solution uses air as the transmission medium to provide a high-performance optical networking system. This approach extends the reach of existing optical fiber networks by using laser, amplifier, and receiver technology to deliver voice and data from rooftop to rooftop. Using dense-wavelength-division-multiplexing (DWDM) technology, Lucent’s Wavestar Optic Air can deliver up to 10 Gbs between locations with distances up to 5 km.
Another trend is the continuing challenge to the wireless Internet by the optical Internet. Nortel Networks, Inc. is creating the optical Internet by extending the optical connectivity from the fiber-optic Internet backbone right up to the desktop personal Computer (PC). Nortel believes that once all optical connections are in place, the optical Internet will provide Internet speeds from gigabits (one billion bits, or 1000 Mbs) to terabits (one trillion bits, or 1000 Gbs). Current wired and wireless Internet speeds are measured in megabits (Mbs). Strange as it may seem, while promoting the fully optical Internet, Nortel Networks has just recently announced its “Wings of Light” strategy that claims to unite both the optical and wireless Internets. This strategy will try extending the benefits of Internet protocol (IP) to wireless networks and devices by unifying IP, optical, and wireless capabilities.
This proposed unification would result in a redefining of how services are delivered and
managed (Fig. 3).
It addresses three primary aspects:
• Applications and Appliances - Nortel’s goal is to ensure interoperability of its Infrastructure and Serviceware with popular access devices and content providers for wireless users.
• Serviceware - Integrates the infrastructure to the applications. Enables a new array of revenue services to be deployed and managed.
• Infrastructure - Converges multiprotocol wireless access/aggregation with IP core networking, transported on optical rings.
3. Nortel Networks’ three-tiered wireless unification.
Aimed at the 3G marketplace, the goal of the "Wings of Light " program is to help wireless service providers speed time to service and time to profit. While an ambitious goal, it may well be within the reach of Nortel, who has now been selected to build out its new Universal Mobile Telecommunication System (UMTS) by six 3G operators: BT Cellnet in the United Kingdom, Cegetel in France, Airtel and Xfera in Spain, TMobile in Germany, and AT&T Wireless in the US.
The rapidly growing development of optical-wireless-communication technology is just another reason why wireless-systems designers should understand the basics of fiber-optic technology, from components through system-level design. Conversely, optical designers are gaining much from their wireless counterparts, especially in terms of new modulation techniques that allow even greater data transmission on the same fiber lines. This give-and-take between electronic and optical engineering disciplines and technology domains will be of great benefit to both the wireless and optical industries, and ultimately to their customers.
(This article first appeared in Wireless Systems Design magazine - April 2001)