Fresnel Zone and Line-of-Sight Considerations

Fresnel Zone and Line-of-Sight Considerations
Line of sight in microwave includes an area around the path called the Fresnel zone. The Fresnel zone is
an elliptical area immediately surrounding the visual path. It varies depending on the length of the signal
path and the frequency of the signal. The Fresnel zone can be calculated and must be taken into account
when designing a wireless link. Any object within the Fresnel zone attenuates the transmission path
between two points. The maximum radius of the Fresnel zone can be calculated by the following formula:
(5-3)
where d is the distance in miles and f is the frequency in GHz.
For a 5-mile link at 2.4 GHz, the radius is 31.25 feet.
Line of sight means a direct, unobstructed path exists from the transmitter to the receiver. This usually
means in any wireless network that the transmission will suffer less degradation than it would if an object
(s) was obstructing that path. Line of sight is the best possible configuration for transmission on 802.11
networks.
Non line of sight means the radio link is blocked. However, with proper engineering, it is possible to
receive 802.11 services without having a direct line of sight to the service provider's transmitter. This term
usually applies where the service provider has deployed its transceivers in a cell network where a
backbone services individual cells. If a subscriber is in a location that is non line of sight, he or she will
not be served by the WISP. To reach that prospective customer, the service provider would have to
deploy a new, costly base station.
One alternative to a new base station would be any-point-to-multipoint technology or an ad hoc peer-topeer
network. In the case of an any-point-to-multipoint network topology, any node already in the network
can be used as a relay point to reach the central site (see Figure 5-4). If location 5 is within the line of
sight of location 2, for example, node 2 will start functioning as a repeater by simply installing a wide
focus antenna connected to its port B. At the new subscriber site (location 5), a transceiver is installed
with a directional antenna pointing at location 2.[8]
Figure 5-4: Using any-point-to-multipoint technology to reach a non-line-of-sight subscriber
Ad hoc peer-to-peer technology, also known as mesh networks, can also provide a cost-effective means
of providing service to non-line-of-sight locations. Single long radio links are replaced with several shorter
ones that are less susceptible to noise and multipath. In an ad hoc peer-to-peer network, something as
simple as a subscriber device (a handheld personal digital assistant [PDA], cell phone laptop, and so on)
can be used as a repeater to reach an AP or base station. The downside is that current client adapters
must use new client software to control the routing function of the subscriber device and change it from
infrastructure and ad hoc as needed. The cost of APs, base station technology, and wireless routers is
becoming less expensive as time goes by. Therefore, a service provider's ability to reach subscribers
increases with time. The same goes for potential subscribers if they want to provide the equipment to
receive wireless broadband. Not having line of sight to a base station or AP should not prevent them from
receiving the benefits of wireless broadband.