In today's modern world, the interconnection and interfacing of differing technologies are becoming commonplace. Therefore, gaining an understanding of how these interconnects and interfaces interact is critical to successful system design.

This article discusses the use of fiber optics in conjunction with the wireless local area network (WLAN) standard 802.11a to distribute RF signals. Such a system is an attractive option for high data rate, short-range links, where deploying optical fibers all the way to the customer premises is too expensive or otherwise impractical.

The primary objective of this article is to investigate the technical feasibility of using an integrated optical and wireless infrastructure capable of delivering broadband multimedia traffic to subscribers in remote areas. In such a scheme, the fiber is used to route the broadband modulated optical signals to base stations where the RF signals are detected and transmitted to client stations. The use of RF over fiber allows a significant reduction in the complexity and costs of remote base stations. It also provides an inexpensive method for system upgrades, since most of the signal processing functions would be done at the central office and not at each individual base station.

An experimental wireless over fiber system operating in the Unlicensed National Information Infrastructure (UNII) band at 5.15 GHz to 5.35 GHz was assembled and tested to demonstrate the potential of this technology. The proof-of-concept system, as well as the measured results, will be discussed in this article.

The use of fiber optics to transport digital signals is quite common. However, the transmission of analog RF signals has been limited by the linearity constraints in modulating/demodulating devices, and by the distortion effects created by the optical link. Advances in fiber optic technology now allow modulating laser devices with RF signals beyond 10 GHz. Utilizing optical devices that operate at high frequency to carry WLAN 802.11a signals could enable delivery of broadband multimedia traffic to subscribers in many wireless scenarios^1-5. WLAN 802.11a systems can provide very high-speed Internet access (up to 54 Mbps)^6-8 for indoor environment such as public buildings, shopping malls, and airports.

The advantages of 802.11a over wired LAN includes fast flexible radio deployment. Combining fiber distribution and a WLAN multiplies the capacity of the system by a large factor, and also solves several problems related to deploying WLANs in outdoor environments in economical and flexible ways. The maintenance and upgrades of the system are simplified since the processing units for many cells are gathered under one roof.

Figure 1 illustrates the basic concept. In figure 1, a base station (also known as an access point) communicates with several wireless terminals using the 802.11a air interface. The base station is connected to a central processing node using a fiber optic link. A complete system may consist of many cells like the one shown here. Adjacent cells operate on different frequencies with eight-channel frequency reuse patterns. All signals from all base stations are processed at a central point, and the RF signals are transported back and forth between the base stations and the processing point on fiber optic links. In the uplink direction (from base station to processing node), a semiconductor laser converts the RF signal to an optical signal to be transmitted over the optical fiber.

In the downlink path (from the processing node at the central office to the base station), an optical detector at the base station converts the optical signal to an RF signal. The access point also needs a power amplifier to provide the needed effective isotropic radiated power (EIRP). The 802.11a standard for North America specifies three power levels for its three 100 MHz bandwidth. The first 100 MHz band (5.15 GHz to 5.25 GHz) is restricted to a maximum output power of 50 mW. The second 100 MHz band (5.25 GHz to 5.35 GHz) is restricted to a maximum power of 250 mW, and the third 100 MHz band (5.725 GHz to 5.825 GHz) is intended for outdoor applications, and is restricted to a maximum output power of 1 W⁸. Figure 2 shows the channelization of the lower and middle bands of the UNII.

The North American implementation of the 802.11a standard also specifies channel spacing of 20 MHz, and guard band spacing of 30 MHz at the band edges to meet the Federal Communication Commission (FCC) and Industry Canada (IC) spectral mask requirements. Figure 3 shows the spectral mask of 802.11a.

One of the main advantages of the 802.11a air interface is its robustness against multipath fading because it uses orthogonal frequency division multiplexing (OFDM)^9-12. Also, merging WLAN and fiber optic technologies would allow the entire digital signal processing functions — such as modulation and demodulation, and frequency assignment — to be performed at the central office. Moving the digital signal processing functions, as well as the control functions, to the central office simplifies the base-station design and reduces its cost significantly.

The goal was to determine whether data transmission using the 802.11a standard could tolerate the distortions caused by a fiber optic link, particularly those caused by the laser. The equipment used in this investigation is shown in figure 4. In this setup, an 802.11a access point was connected to an optical link that consisted of a laser, two meter span of single mode fiber, and an optical detector. The laser is a source emitting at 1.5 µm and the detector is a typical industry standard receiver. Both are specified as having a bandwidth of 300 kHz to 6 GHz.

The overall frequency response of the optical fiber link was measured using a network analyzer operating over the range of 400 MHz to 10 GHz. The transmission, as a function of frequency, is shown in figure 5. Compared to the transmitted signal, the RF response decreases by less than 1 dB over the 802.11a 5 GHz to 6 GHz band of interest. Of course, the variation in the optical link frequency response is even smaller over any given RF channel that is only 20 MHz wide. The response is -9 dB near 5 GHz.

The output of the optical detector was connected to a spectrum analyzer to monitor the 802.11a signal after being carried over the fiber. The output of the optical link was also connected to an antenna to establish a radio link with another laptop computer. Figure 6 shows the test setup used to transfer data over fiber between the access point and the remote laptop computer equipped with a 5 GHz cardbus adapter.

To establish a baseline for the experiment, the RF spectrum of the signal transmitted from the access point towards the optical link was measured. The access point was configured for a 54 Mbps transmission rate, and a 5.28 GHz channel frequency. Figure 7 shows the measured spectrum at the input and output of the two-meter optical link.

From the measured results, a drop in the signal level by about 8.5 dB is observed in the central part of the 20 MHz channel. This drop is in agreement with the response of the optical link shown in figure 5. It can also be observed that there is no significant degradation in the shape of the channel response. This observation is further confirmed by a successful transfer of a video file (53 MB) to and from the access point. Figure 8 shows the RF spectrum at a distance of 0.5 meters from the transmitting antenna. The measured results indicate a drop in the signal level due to the propagation path-loss in the test vicinity, but the shape of the channel spectrum did not degrade.

With increased distance between the transmitting and receiving antennas, the shape of the RF spectrum starts to degrade. To compensate for the optical link losses, and to maintain the system- required bit-error rate (BER), an RF amplifier is required to boost the signal level at the output of the optical link. Once the transmission of the 802.11a signal-over-fiber was validated, another optical-link was added in the reverse direction, as shown in figure 9.

Using this test setup, a successful transfer of a 53 MB video file to and from the access point was accomplished. The transfer rates were not affected by the use of the fiber optic link in these tests. This experiment demonstrated that the optical links were able to successfully transmit and receive WLAN 802.11a signals.

To investigate the impact of transmitting more than one RF channel over one fiber, the experiment shown in figure 10 was implemented.

In this experiment, one channel was obtained from an access point and the other channel was obtained from a vector signal generator. The configuration is shown in figure 11.

The two channels were combined and applied to an optical link. The output of the optical link was applied to a spectrum analyzer to investigate any intermodulation that might occur due to carrying multiple channels. The measured results of the optical link output are shown in figure 12.

From the obtained results, there was no apparent intermodulation distortion due to carrying more than a single channel of OFDM signals over the optical link.

A test setup using commercial off-the-shelf components was assembled and tested successfully to carry 802.11a RF signals over fiber optics. The components used in the fiber optic link do not affect adversely the data transmission. Experiments where two OFDM channels (using the 802.11a standard) were combined to modulate the laser of a fiber-optic link have demonstrated that more than one channel of 802.11a signals can be carried over a fiber-optic link without concern regarding intermodulation distortion.

Enhancing WLAN systems by fiber extensions could provide a good solution for both indoor and outdoor broadband access, while improving spectrum efficiency and robustness against multipath fading. Also, it provides a significant cost reduction and simplification to the wireless base station for future generation wireless systems such as 3G and 4G.

1. I. Haroun, G. Chan, R. Hafez, L. Bouchard, and L. Boucher, Feasibility Study of Radio Over Optical Transmission System for Increased System Efficiency, submitted for publication.

2. Al Raweshidy, H. S. Komaki, Radio Over Fiber Technologies for Mobile Communications Networks, (Artech House: Norwood, Mass., 2002).

3. Hossein Izadpanah, “A Millimeter-wave Broadband Wireless Access Technology Demonstrator for the Next-Generation Internet Network Reach Extension,” IEEE Communications Magazine, September 2001.

4. K. Morita, H. Ohtsuka, “The New Generation of Wireless Communication Based on Fiber-radio Technologies,” IEIC Transaction Communications, Vol. E76-B, No. 9, September 1993.

5. J. Namiki, M. Shibutani, W. Domon, T. Kanai, K. Emura, “Optical Fiber Basic System Design for Micro cellular Mobile Radio,” IEIC Transaction Communications, Vol. E76-B, No. 9, September 1993.

6. Institute of Electrical and Electronics Engineers Inc. Get IEEE 802 Web site: http://standards.ieee.org/getieee802/

7. Institute of Electrical and Electronics Engineers Inc. 802.11 Wireless Local Area Networks Group Web site: http://grouper.ieee.org/groups/802/11/

8. Bob O'Hara and Al Patrick, The IEEE 802.11 Handbook: A Designer's Companion (IEEE Press, 1999).

9. Juha Heiskala and John Terry, OFDM Wireless LANs: A Theoretical and Practical Guide (Sams Publishing, 2002).

10. R. Van Nee, J. Prasad, OFDM For Wireless Multimedia Communications (Artech House Publishing, 2000).

11. L. J. Cimini, “Analysis and Simulation of Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing,” IEEE Transaction Communications, Vol. COM-33, No. 7, July 1985.

12. S. B. Weinstein, P.M. Ebert, “Data Transmission by Frequency Division Multiplexing Using the Discrete Fourier Transform,” IEEE Transaction Communications, Vol. COM-19, No. 5, Oct. 1971.

Ibrahim Haroun is a senior research engineer at the Communications Research Centre Canada (CRC at www.crc.ca) where his current research activities include study of WLAN 802.11a and 802.11b over optical fibers. Previously, he worked in the telecommunications industry where he designed and developed RF transceivers for TDMA Base-stations and cellular phones. He holds a M.Sc.(EE) degree in high frequency and antenna engineering from the University of Manitoba (www.umanitoba.ca). He is also a registered professional engineer. He can be reached at ibrahim. haroun@crc.ca.

François Gouin, completed a Ph.D. at McMaster University (www.mcmaster.ca) in solid-state physics for work on semiconductor lasers. In 1988, he joined CRC where his current research interests include the study of MSM photodetectors, their application as microwave switches, and the integration of optoelectronic and optical signal distribution components. He can be reached at francois.gouin@crc.ca.

The authors would like to thank Prof. R. Hafez from the Systems Engineering Department at Carleton University (www.carleton.ca), L. Bouchard of the Wireless Applications and Systems Research at CRC, and J. Noad of the Optical Materials and Components group at CRC for useful discussions. Thanks also to Miteq Inc. (www.miteq.com) for making a fiber-optic link available and Agilent Technologies Inc. (www.agilent.com) for the vector signal generator.

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