d Kam Y. Lau

Building the FO Infrastructure

for Wireless Communications

A look at device and system criteria for modern analog fiber-optic links

he next generation of communi- cation systems will provide con- sumers with a host of new and T improved voice, video, and data

services. Rapidly developing wireless radio systems are already contributing to the pro- gress, giving consumers the luxury of teth- erless access to telephone conver sation, on-line computing, and cable-television. In- deed, wireless access to high quality infor- mation and entertainment is quickly becoming a key component of the informa- tion superhighway. In that context, optical fiber links, which have the bandwidth for providing users with the service of their choice, are an excellent connecting infra- structure for providing uniform radio cover- age in wireless networks. Indeed, fast improving analog FO links will serve such networks well. In this article, we will ana- lyze the requirements for such a system.

Historical Background Since the development of optical fiber and the semiconductor laser, fiber-optic links have found application as both digital and analog communication channels. A natural use of these low-loss, high bandwidth opti- cal links is for long-haul (10 km) and/or high speed (1 Gb/s) transmission of digital sig- nals. In this application, the light output

13 8755-3996/95/$4.000 1995IEEE July 1995

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from a laser is simply modulated on and off, such that each light pulse represents a bit of information. These types of links are used in telecommunication systems and for high- speed computer interconnects. Digital fiber- optic l inks have been commercially available for over 15 years, and during this time researchers have continually improved their reliability, speed, and cost.

More recently, fiber-optic links use lin- ear optical devices for the transmission of analog signals. In this scenario, a high-fre- quency analog signal is transmitted in the time-varying intensity of the light output from the optical transmitter. The primary issues in characterizing the suit ability of optical devices for analog signal transmis- sion are the linearity of the light output with respect to the modulation signal, and the signal-to-noise ratio after transmission through the link.

Analog fiber links have found commer- cial application for cable television (CATV) video signal distribution [l]. Analog CATV transmission using fiber-optics was actually proposed in the early 1970’s, but device linearity was so poor that the idea was ruled out immediately. Recent advances in linear fiber-optics technology have made this ap- plication feasible. Many analog video chan- nels (in the frequency range 50-550MHz) are sent over a fiber link to a head-end distribution point where they are distributed to subscribers over coaxial drops. The low- loss optical fiber allows for transmission of the video signals over a long distance ( 5 km) without signal repeaters. Analog fiber links have also found application in military sys- tems for the delivery of signals to and from remote satellite locations, and as feeders for phased-array radar systems [2,3], where many microwave antennas must be driven with synchronous signals in the GHz fre quency range. Both CATV and military ap- plications of analog fiber links are very de- manding in terms of device performance (Le., optical transmitter linearity and noise). Hence, analog fiber links are historically high cost products for these applications. This high product cost is in sharp contrast to the cost and device performance required for devices used in the emerging area of fiber- fed wireless networks.

Fiber-optic Backbones The goal of a wireless radio system is to provide acceptable communications to mo- bile and spatially distributed users (Fig. l).

A primary difficulty in achieving this goal is creating a network with uniform radio coverage. This problem can be severe in urban environments where buildings and other obstructions shadow the radio signals. This situation is unfortunate, because urban settings and buildings are areas of high con- sumer demand for wireless services. The classic solution to this radio coverage prob- lem is simply to build more antenna sites, or base stations, to cover a given area. For example, this approach has been imple- mented by cellular telephone service provid- ers to provide more coverage and capacity in many popular urban areas. Although the addition of new base stations does improve coverage, the installation of the expensive radio hardware, and the required real estate for the cell site, may be prohibited by cost.

An alternative, and more cost effective solution, is to implement analog fiber-optic links as feeders to remote antenna sites (called ‘microcells ’) within the coverage area of a single cell site [4-71. If we consider a cell covered by a single base station (Fig. 2), the radio coverage within the cell can be improved by deploying several remote an-

tenna sites within the cell that are linked to the base station via analog fiber optic links. In this way, each antenna site simply con- sists of an antenna, an amplifier, and an optical transmitter. Using optical fiber for this application provides advantages that have been exploited in digital links for many years, including low-loss, wide bandwidth, and immunity to electro magnetic interfer- ence. The resulting fiber-fed distributed an- tenna network provides blanket radio coverage to the area without the need for any new base stations. The same concept can be applied to solve in-building radio coverage problems, where radio signals cannot ade- quately reach areas deep into the building. In this case, fiber-fed antennas can be placed at strategic locations within the building, and the fiber is routed to the roof where the radio coverage is excellent. Also, in-build- ing wireless LAN and PBX services can be supported by the fiber-fed distributed net- work without interfacing with the outside base station. For both the outdoor fiber-fed micro cellular systems, and the indoor ‘pi- cocel lular’ systems, the feasibility of the fiber-optic approach is very sensitive to the

n ‘ Optical Fiber Backbone I 7 . Future wireless personal communication network with fiber-optic backbone infrastructure.

Optical Fiber

Base Station

2. Fiber-fed distributed antenna network to provide uniform radio coverage to a given area.

14 Circuits & Devices

cost of the fiber links. In fact, the entire appeal of using fiber-fed antennas versus construction of new base stations is a potential savings in cost. Due to the cost- sensitive nature of the wireless application, the design emphasis is often on the tradeoff between device cost and performance. This issue is typically not empha- sized in traditional fiber-optic ap- plications such as the telephone and CATV industries because in these systems, the cost of the link is shared among many consum- ers. The story is different for fi- ber-fed microcells, where we desire cost effective deployment of many fiber-links to provide blanket radio coverage.

2 M 6M

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I

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Commercial Device Technology There are three competing tech-

3. Narrow-band, millimeter-wave optical transmitters based on the technique of resonant modulation.

nologies for fiber-optic support of wireless radio systems: external modulation, digital RF transport, and direct current modulation of semiconductor lasers. In external modu- lation, a constant light source is sent into a device where the light transmission through the de\ ice is modulated. External modula- tion has the advantage that the optical output (transmitter) can be located at a central loca- tion (like a base station), with the external modulator serving as the remote antenna site [8]. Also, external modulators, in conjunc- tion with rf-predis tortion circuits and high power lasers, can achieve a very high dy- namic range. AI though direct current modu- lation requires a semiconductor laser transmitter at each antenna site, it is typically a more simple and cost-effective solution.

Digital rf transport is a scheme in which the entire radio band of interest is down converted to an intermediate frequency, and then digitized by a high speed analog- to-digital converter. Once this is done, a digital fiber link is used for the remote transport of this digit- ized radio band to the base station, where it can be converted back into analog form, or processed di- rectly using digital techniques. This scheme has the advantage that it implements the well devel- oped digital fiber-optic technol-

ogy, which can transmit the signals many kilometers while maintaining an excellent bit error rate. Also, the analog linearity of the entire link will be determined by that of the digitization electronics and not the fiber link. Since the entire radio band must be digitized, the primary limitation of this tech- nique is the required speed of the a/d con- version process. Recent progress in high-speed a/d converters has made this scheme very competitive with the analog scheme for support of narrow ( MHz) radio bands. As future higher bandwidth radio systems develop, it will be interesting to see which technology will be more practical.

tivation for implementing these low cost. commercially available devices in present wireless systems.

Millimeter-Wave Optical Transmitters The transmitters discussed above may be acceptable for the existing wireless cellular systems that operate in the 800 MHz-2.4 GHz frequency range. However, as de- mands for wireless services increase, the need for larger radio bandwidth may drive wireless systems to operate at higher carrier frequencies, where bandwidth is more plen- tiful [9]. The development of low cost, high speed analog optical transmitters is a major

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There are a variety of commer- cially available analog optical transmitters based on direct cur- rent modulation of semiconductor lasers. They run the gamut in terms of both price and performance. The highest bandwidth commer- cially available device has a modu- lation bandwidth of about 20 GHz and costs approximately $20,000. At the other end, typical low band- width (1-2 GHz) Fabry-Perot la- sers cost below $100 each in quantity. Although modulation bandwidth is only one of several performance criteria, the cost/per- formance range of commercially available devices is large, and even the lowest cost devices have modulation bandwidths in the GHz frequency range. Given that most present wireless services op- erate in frequency bands less that about 3 GHz, there is a strong mo-

Noise Floor IP3 Point -1 40

4 , , , , . , , I . , .

-80 -60 -40 -20 0 20 40 264.6M Input RF Power (dBm)

4. Illustration of a typical two-tone dynamic range measurement with signal, noise, and intermodulation distortion (IMD) terms shown.

research challenge. This problem can be simplified by noting that high frequency radio communi- cations typically take place in a relatively narrow bandwidth around a high frequency carrier. For example, a 100 MHz radio band centered at 50 GHz occu- pies a fractional bandwidth of only 2 percent. Keeping this in mind, consider a riar.row4and optical transmitter that operates at high (20 GHz) frequencies.

A potentially low cost, nar- row-band. high-frequency opti- cal transmitter is based on the technique of resonant modulation [ 101. A semiconductor laser, cleaved to a given length (L), has

July 1995 15

a set of allowed optical modes that are sepa- rated in frequency by c/2nL, where n is the effective index of refraction of the laser, and c is the speed of light. By properly choosing the length of the laser, this mode separation can be chosen to lie within the millimeter- wave frequency range. For example, if n = 3.5 and L = 850 pm. then the optical mode spacing is about SO GHz. By modulating the gain of the laser (i.e., the current) at this intermodal frequency, we observe a signifi- cant modulation of the light output from the laser. This phenomenon results from the coupling of the optical modes of the laser, and is historically referred to as ‘mode- locking.‘ In terms of modulation response, the effect appears as a narrow-band reso- nance centered at the intermodal frequency.

Figure 3 shows the technique of resonant modulation and a typical measured modu lation response at low and high frequen cies. Note that for this device, the conventional low-frequency response is limited to ap- proximately 1 GHz, but the resonant modu- lation effect produces a 100 MHz bandwidth response at a center frequency of 45 GHz.

There are several important points to be observed. First, this technique i s a way of utilizing ordinary devices, with no special processing or design for high frequency per- formance, to transmit high-frequency ana- log signals. Second, by cleaving the device to the appropriate length, we can set a nar- row-band optical transmitter at an arbitrary center frequency. These devices show good potential for the support of future high fre- quency wireless systems. Required Device Performance A major factor in the cost of an analog optical transmitter is the required linearity of the optical modulation signal. This linearity requirement i s typically speci- fied by a two-tone dynamic range measurement (Fig. 4).

Two equal ampli tude sinusoidal signals at frequencies fi and fz are used to modulate the link. The third order nonlinearity of the link introduces intermodu- lation distortion (IMD), resulting in output signals at 2fi-f2 and 2f2-f1, as shown in the inset of the figure. This distortion is of particular interest since the im- pairment lies ‘in-band’ near the signal channels of interest. If we graph the output signal, IMD,

and noise powers as a function of the input signal power to the link, we obtain the two- tone dynamic range plot as shown. Note that there is a range of input powers over which the output signal is above the noise floor, and the distortion below the noise floor. When specified in a 1 Hz noise bandwidth, this range of powers is called the two-tone dynamic range of the optical link, and is a figure of merit for the linearity of the link. Also, the input power for which the IMD equals the signal power is called the IP3 point, and is simply related to the dynamic range discussed above. With this under- standing of the standard characterization of analog optical transmitters, we can address the question: what is the required dynamic range of a fiber-optic antenna remoting link in a practical microcellular application?

For wireless systems, the two-tone speci- fication of link linearity overestimates the required link performance [ I 11. This is be- cause the two-tone specification corre- sponds to the rare and unfortunate situation where two high power wireless users happen to be assigned to adjacent frequency chan- nels. More practical values of dynamic range requirements can be obtained by per- forming a statistical study of wireless access in a microcellular environment. In the analy- sis, we focus our attention on the uplink (from remote antenna to base sta tion) be- cause the downlink (from base station to remote antenna) carries a narrow range of signal powers, and the performance is typi- cally limited by the linearity of the power

amplifier at the remote antenna site. The simulations are based on a statistical model of the received power from mobile users randomly located between 20 and 1200 me- ters from the microcell antenna site. If we assume a simple ( l/d)4 path loss, the range of received powers at the antenna is approxi- mately 70 dB. This ( l/d)4 model provides a lower bound on the received signal power in a typical urban radio environment. The inset in Fig. 5 shows a line-of-sight (LOS) radio measurement at 900 MHz in New York City from [ 121, with the ( 1/d)4 lower bound illus- trated. For non-LOS paths, existing mobile radio handsets typically have some limited power control capability that can compen- sate for the effects of shadowing. Although this is a simple model of the radio environ- ment, the objective of this analysis to com- pare the requirements of the analog fiber links in this application relative to the ‘spur- free’ specification.

Figure 5 shows the results of such a simu lation based on the AMPS wireless Stan dard, which allocates 30 kHz of bandwidth per user and requires an 18dB canier-to-in- terference ratio (C/I) for acceptable service. If the system designer were to stick to the ‘spur-free’ condition imposed by the two- tone specification, the parameters above im- ply a required dynamic range of about 120dB. By considering many configura- tions of users within the microcell, and cal- culation of the resulting IMD introduced by the fiber link, we can determine the percent- age of calls that do not maintain the required

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Fiber-fed Microcell Simulation Results 5 , , . , . , . , . , * , . , . ,

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5. Simulation results for analog link performance in a typical microcel- lular environment. The inset shows the received power in a typical urban microcellular environment from [12].

18 dB of C/l after the link; This blocking probability is plotted as a function of the link two- tone dynamic range in Fig. 5 for 5 , I O and 20 users within the microcell.

Increasing the dynamic range of the link clearly reduces the percentage of blocked calls. Also, increasing the number of users within the cell increases the required dynamic range to reach a given blocking prob- ability due to the increased number of intermodulation products. We see that by ac- cepting avery small probability of blocked calls (0.5%) that the required link dynamic range is significantly reduced from 120 dB to the 90 dB range. This concept of allowing a small

16 Circuits & Devices

Table: Comparing the noise, distortion, and dynamic range performance of various analog optical transmitters.

High performance CATV distributed feedback (DFB) laser

Low cost Fabry-Perot laser

Optical Transmitter

-155 +25 120

-115 +20 90

Noise Floor IP3 Point Dynamic

(dB-Hza3) 1 (dBIHz) 1 (dBm) ~ Range

-100 Millimeter-wave optical transmitter based on resonant modulation

+5 70

degradation in network performance in ex- change for a 30 dB reduction of dynamic range requirement has substantial implica- tions for the use of low-cost optical transmit- ters in this appli cation.

With this insight into the required dy- namic range of analog optical transmitters for wireless applications, we now compare the performance of the various analog opti- cal transmitters discussed above. The Table compares the noise floor, IP3 point, and two-tone dynamic range of a typical CATV Distributed Feedback (DFB) laser, a low- cost Fabry-Perot laser, and a millimeter wave optical transmitter based on resonant modulation. Note the wide range of perform- ance in noise, linearity, and dynamic range between the devices. To achieve the absolute spur-free operation, the conventional CATV optical transmitter is required. This conclusion is consistent with the approach taken by cellu- lar service providers in the past. If we accept the slight degradation in network service (0.5% blocking probability, which is accept- able even for wired networks), we see that the low cost commercially available Fabry-Perot lasers may be appropriate for use in both out- door microcellular and indoor picocellular systems. For the millimeter-wave optical transmitters based on resonant modulation, the dynamic range is very modest by conventional CATV measures. Device structures and tech- niques to improve the dynamic range of these transmitters is a significant future research challenge.

Future Directions The majority of consumer wireless services at the present time are concentrated in radio bands below 3 GHz. The above analysis has argued that low-cost commercially available semiconductor lasers have sufficient per-

formance for the support of wireless systems in these bands. Given that these relatively low frequency radio systems will continue to dominate in the near future, the existing challenge is to implement practical fiber-fed radio systems based on low-cost devices. In addition to the analog fiber systems empha- sized above, the technique of digital RF transport using a/d conversion and digital fiber links is expected to be a very competi- tive scheme for the support of wireless serv- ices.

Incorporating both analog and digital schemes in fiber-fed radio networks intro duce a host of new research issues. One issue is the development of optimum networkpro- tocols for the assignment of mobile users to a particular antenna or group of antennas. Also, techniques for interfacing fiber radio systems with existing wired networks is ex- pected to be an important issue.

In summary, low-cost analog fiber-optic networks may be useful to meet the rapidly expanding demand for wireless communi- cation services. This effort involves the de- sign of networks that can be supported by low-cost, commercially available devices, and the development of narrow-band, high- frequency optical transmitters for future wireless systems. These technologies may have a significant impact on the realization of future wireless personal communication networks. CO

David M . Cutrer is a Ph.D. candidate, John B . Georges is a post doctoral research fellow, and Kam Y . Lau is a professor of electrical engineering, all at the University of California at Berkeley.

References 1. J. A. Chiddix, H. Laor, D. M. Pangrac, L. D. Williamson, and R. W. Wolfe, “AM Video on Fiber in CATV Systems: Need and Implementa- tion,”IEEE Journal on Selected Areas in Commu- nications, 8(7), pp. 1229-1239, September 1990. 2. W. I. Way, “Fiber-optic Transmissions of Mi- crowave 8-Phase-PSK and 16-ary Quadrature- Amplitude-Modulated Signals at the 1.3pm Wavelength Region,” Journal oflightwave Tech- nology, 6(2), pp. 273-280, February 1988. 3. W. Ng, A. A. Walston, G. L. Tangonan, et. al., “The First Demonstration of an Optically Steered Microwave Phased Array Antenna Using True- Time-Delay,’’ Journal of Lightwave Technology, 9(9), pp. 1124-1131, September 1991. 4. W. I. Way, “Optical Fiber-Based Microcellular Systems: An Overview,” IEICE Transactions on Communications, E76-B(9), pp. 109 1 - 1 102, Sep- tember 1993. 5. T. S . Chu, and M. J. Gans, “Fiber Optic Micro- cellular Radio,” IEEE Transactions on Vehicular Technology, 40(3), pp. 599-606, August 1991. 6. L. J. Greenstein, N. Amitay, T. S . Chu, et. al, “Microcells in Personal Communications Sys- tems,” IEEE Communications Mags-ine, pp. 76- 88, December 1992. 7. M. Shibutani, T. Kanai, W. Domom, et. al. “Optical Fiber Feeder for Microcellular Mobile Communication Systems,’’ IEEE Journal on Se- lectedAreas in Communications, 11(7), pp. l l 18- 1126, September 1993. 8. H. Ogawa, K. Horikawa, and Y. Nakas uka, “Cascaded External Optical Modula tion Link for Radio Distribution,” 1994 MTT-S International Microwave Sympo sium Digest, pp. 487-490, San Diego, CA, 1994. 9. H. Ogawa, D. Polifko, and S . Banba, “Millime- ter-Wave Fiber Optics Systems for Personal Ra- dio Communication,” IEEE Transactions on Microwave Theory and Techniques, 40( 12). pp. 2285-2293, December 1992. 10. J. B. Georges, M.-H. Kiang, K. Hep pell, et. al, “Optical Transmission of Nar row-Band Mil- limeter-Wave Signals by Resonant Modulation of Monolithic Semiconductor Lasers,” Photonics Technology Letters, 6(4), pp. 568-570, April 1994. 11. D. M. Cutrer, J. B. Georges, T. H. Le, et. al. “Dynamic Range Requirements for Optical Transmitters in Fiber-Fed Micro cellular Net- works,’’ Photonics Technology Letters, 7(5), May 1995. 12. A. J. Rustako, N. Amitay, G.J. Owens, and R.S. Roman, “Radio Propagation at Microwave Frequencies for Line-of-Sight Microcellular Mo- bile and Personal Communications,”lEEE Trans- actions on Vehicular Technology, 40(1), pp. 203-210, February 1991.

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