Optical Wireless Communications
By Dick Walter, Laser Wireless, Inc.

Everyone today is looking for ways to increase the amount of information that can be sent and received and the demand will only increase in the future. There are limitations on some technologies like microwave and spread spectrum, so this article will examine the capabilities of communications by way of a beam of light.

The wireless communications industry has not yet evolved to what it will become with the integration of optical network technologies. There are several advantages to using optical communications: there are no license requirements by the federal government, optical communications can handle extremely high bandwidths, and the systems are easy to install and relocate as needed.

Even with these advantages, limitations for optical communications do exist. Optical communications are affected by fog. Therefore, the design of the equipment will affect the range of the transmission. If the manufacturer includes an electronic tracking system, the beam of light can be made smaller while still maintaining alignment. If the manufacturer does not include an electronic tracking system, the system must then open the beam width to maintain alignment. In this process, the energy density decreases, which then diminishes the range of the system. Atmospheric conditions also affect these types of systems so care should be taken when choosing an optical communications system.

Additional factors should be considered to ensure that system capabilities satisfy the operator’s desire for particular applications. Bandwidth capabilities are very diverse depending on the manufacturer’s equipment. There are various wireless optical systems, some of which have data ranges from 1 to 10 mega bits per second, others from 45 to 100 mega bits per second and still more systems that handle 100 to 155 mega bits per second. There are even a few systems that can handle everything from 1 Mbps up to 155 Mbps. This type of transmission system is especially useful as it provides an opportunity for customers to choose the communications that will be used within their network. A few wireless optical systems on the market today can handle 622 Mbps. In the last quarter of 2000, 1.25 Gigabits systems will be available, simply meaning 1.25 billion bits of information can be transmitted per second.

The best way to describe how optical systems can be used is to compare the cost of installing a fiber cable against the cost of the optical system. An optical system is really a fiber system without a physical fiber. When physical barriers such as a river, highway, railroad, tennis courts, swimming pool, or parking lot restrict the installation of a fiber cable, the very practical solution of wireless optical systems is present.

There are many ways an optical system can meet a customer’s needs. As an example, systems manufactured by Laser Wireless are capable of transmitting any data rate from 1 Mbps up to 155 Mbps. With this bandwidth available, operators can achieve solutions to most every problem. If the operator is trying to transmit 100 Mbps of data and also needs three T1 lines at the same time on the same system, the proper multiplexers at each end can be acquired so that the system will provide the necessary data rate. The multiplexers simply multiplies the data onto one data stream so that it can be transmitted though the laser system. The connection to any laser system is through a fiber connection on the laser back panel. This is usually done through either a FC or SC connection, depending upon whether the system design was intended for outdoor use or not. FC is usually used when an outdoor installation is required.

When considering optical wireless manufacturers, your goal should be to select a manufacturer willing to learn your requirements and who will assist in the selection and design of the multiplexers to ensure your system will operate at its optimum performance level. There are multiple examples that could be given about what might be done on one laser system, including different combinations of video, audio, telephone and data. With proper selection of the multiplexers, any number of different data transmissions can be done simultaneously on the same system. Often times, businesses outgrow their facilities in one building and take ownership or leased space in an adjacent building, say one-half to one mile away. Instead of buying a switch and a server in the new location, an optical system can be used to extend the existing network.

Eye safety is a main concern and should be recognized. Every laser manufacturer that sells a product containing a laser must meet certification required by the Federal government. The energy density emitted from the system on the lens where the light comes out must be less than what would injure someone’s eye if they looked into it. However, under no conditions should anyone stare into an optical system.

One of the most unique benefits to any communication system is the ability to monitor the system while in operation. All systems should have a remote status monitor included. This device, usually located in a control room, provides the 24 Volt DC power to the laser system on the roof. It also gives the operator the ability to connect the laser system to the network by using Telnet, and thereby monitor the performance of the laser system. The remote status monitor also has a phone jack on the back of the box. If a problem occurs, the operator can have the problem completely identified by inserting a standard telephone line into that jack, through which the factory can directly diagnose the problem anywhere in the optical system.

When evaluating wireless optical systems, look for systems designed to be installed by one person. A typical installation includes installing the unit on the first site, bolting it into place, and aiming it at the second site. A retro reflector or corner cube is placed on the front of the lens of the first unit before installation begins on the second site. At the second site, the system is aimed at the first site and turned on. A power level is read on a series of LED’s on the back, which gives the operator the opportunity to align it. After this is completed, the system is tightened down and the installer goes back to the first unit. At the first unit, the retro reflector or corner cube is simply removed and a small adjustment may have to be made. In just a few seconds, the alignment is complete and the system can then be fastened down. Once this is done, the fiber cables can be connected and a complete, one-person system installation will have been performed.

There are many examples of how laser or optical systems can be used. This technology is possibly most useful in a campus environment where buildings are in close proximity to each other. This can also be true for a downtown area, hospital complex, correctional facility or other institution. The advantage of the technology is the fact that it has high bandwidth capabilities and can easily be relocated, a considerable advantage since most hospitals and colleges change the use of their buildings frequently.

Availability is a major concern to operators, or more specifically how fog will affect the optical wireless communication system. We recommend systems be limited to 2.5 kilometers. With our past experience, we can confidently make the statement that an optical wireless system can only maintain a maximum distance of 2.5 kilometers. At this distance, the system will work at its optimum capacity with only a 3-5 percent possibility that the laser beam would be unable to penetrate through the worst fog conditions. This depends on the users specific geographical location in the country. Under normal conditions, there is a sensitivity of approximately 20 db, which means that greater than 99 percent of the signal can be blocked and the system will still work.

Previously it was stated that many operators need 100 percent availability, which is something that is hard to achieve using any form of technology. If fiber is being used, the fiber can be cut. If microwave or spread spectrum radio systems are being used, fog, rain, and snow can affect both. The real solution to 100 percent availability lies in having a laser system operating and using an optical switch, readily available on the market today, and connecting the switch to the spread spectrum radio system. In the extreme circumstances when the laser system will not be able to penetrate trough fog, as an example, the spread spectrum radio system will take the data that was being sent on the laser system and continue sending it. The only noticeable difference will be a decrease from the high bandwidth capabilities of the optical wireless communication system to the 10 to 11 Mbps that the spread spectrum radio system can transfer, but data will still be transferred. The only result of a wireless optical system failure would be a decrease from the high bandwidth of a laser system to the 10 to 11 Mbps available with a spread spectrum system. The data transmission would continue, which should be enough to carry the system in that short period of time that the laser system would not be available.

In the event that the operator has to have 100 percent availability or has a concern about the system's performance, we recommend the purchase of a hot standby or one other unit to be in the possession of the operator. This hot standby is a replacement that can be installed in minutes by the operator with just a few words of instruction from the manufacturer.

As more people become familiar with optical communications, operators are going to become more familiar with the fact that these systems are a real alternative and provide a very practical solution.

About the author

Dick Walter is President of Laser Wireless, Inc with over 15 years experience in the wireless optical communications industry. Mr. Walter can be reached at 877-laser-57 or via email at dwalter@laserwireless.com