Fiber, Wireless, and Free Space Optics: The Ties That Bind

The new industry buzz word these days is wireless. Defined by IEEE standard 8.02.11, wireless refers to the telecommunication technology, in which radio waves and microwaves carry signals to connect communications devices. These devices include pagers, cell phones, portable PCs, computer networks, GPS, satellite systems and handheld personal digital assistants (PDAs). This term evokes the idea of a network detached from wire with a transmission scheme that consists of voice and data quickly whizzing through the air from point A to point B. This idea is partly true. Voice and data can be sent "whizzing through the air," a technology dubbed Free Space Optics (FSO), via lasers and high powered LEDs that transmit through the air rather than through an optical cable. The desire to develop wireless networks lies in the ease of installation, and the rapidity of its technological advances. Breaking away from optical fiber allows a network to go virtually anywhere by eliminating the need to dig ditches and break up streets to install the fiber cable. In addition, over-air wireless transmission is free because wireless optics use the 300 GHz spectrum and above, which includes infrared frequencies, a range that remains unlicensed. Currently, the only regulation on these transmission frequencies is that the radiated power cannot exceed the limits established by the International Electrotechnical Commission or the United States' FDA. Soon, the United States is expected to adopt the IEC standard, creating a global wireless transmission standard.

Typical Wireless Applications

Many applications use wireless technology. The most prevalent applications include those in a local area network (LAN) where right-of-ways are unable to accommodate fiber or copper. The following are the main wireless applications implemented today: 
Last-Mile Access: High-speed links that connect end-users with Internet Service Providers or Satellite services. This applications remains the most popular implementation for wireless technology, eliminating, where applicable, the need for fiber to the curb or fiber to the home. 
Metropolitan Area Network extensions: Used to connect new networks, their core infrastructure, to complete. 
Enterprise Connectivity: Used to connect Local Area Network segments housed within buildings that do not have easily accessible right-of-ways for fiber. 
Fiber Backup: Act as a backup for a fiber based system. 
Backhaul: Wireless system used to carry cellular phone traffic from antenna tower back to facilities wired into the public switch telephone. 
Service Acceleration: Used to provide instant service to fiber optic customers while the fiber infrastructure is being laid.

Fiber Optic Transport in Wireless Networks

Fiber Optic transceivers may be used to connect the Uplink/Downlink equipment to the transmission towers. This allows a greater increase in the distance between the base station and the wireless transmission towers. By incorporating fiber optic links, the system's EMI sensitivity is greatly reduced while reliability and signal quality go up. Figure 1 illustrates a typical use of fiber optic transceivers in a wireless network.

Figure  - Typical use of fiber optic links in a wireless network.