802.16: The Future in Last Mile Wireless Connectivity - Page 3
The Importance of Frequency Bands
Compared with LMDS, 802.16 is a next-generation technology that operates over greater distances, provides more bandwidth, takes advantage of a broader range of frequencies, and supports a greater variety of deployment architectures, including non-line-of-sight operation — a very significant advantage. 802.16 is nominally specified to operate over a 50 km radius and support channels ranging up to the tens of megabits.
No single 802.16-compliant product will operate over the entire 2-66 GHz frequency range. In fact, that frequency range represents most of the radio communications spectrum. So, why has the IEEE defined 802.16 so broadly? The reasons are a combination of physics, regulatory issues, and user requirements.
Radio signal propagation depends on its frequency. The lower frequencies in the 802.16a standard do not require line-of-sight to work. Easing the requirement for line-of-sight between transmitter and receiver widens the range of feasible product offerings. For example, the roof of your home may be too low for line-of-sight service to work, but a non-line-of-sight implementation would enable carriers to deliver wireless broadband directly to consumers.
Vendors of wireless products are very sensitive to regulations. In the US, the FCC is responsible for the allocation of all radio frequency bands. Other countries have their own equivalent regulatory authorities. In addition to defining how the frequency spectrum is divided into bands and prescribing their usage, the FCC also specifies if a license is required to transmit on a particular band. It may also limit the power of a transmission.
The regulations are designed to minimize interference and maximize the overall utilization and usefulness of the spectrum. The ability to purchase a license for a particular piece of spectrum assures a carrier that there will be no signal interference from other carriers.
Wireless networks deployed by carriers operate at a frequency and power level that allows the signal to cover a wide region. Wireless devices intended to operate inside an enterprise would use an unlicensed frequency band and power level designated for short-distance communications. While the lack of a license requirement allows the enterprise to avoid delays and costly paperwork, there is a chance of interference from other kinds of devices that emit (intentionally or not) at the same frequency. Microwave ovens which emit over a broad spectrum are well known offenders, while 2.4 GHz wireless telephone handsets may also cause problems.
Because of the decision to define 802.16 to operate across such a broad frequency range and in many different countries, the standard supports a variety of physical layers. For example, for 10-66 GHz line-of-sight operation, the base station uses Time Division Multiplexing (TDM). This technique allocates timeslots on a single frequency to address each customer’s receiver separately as a way to share the bandwidth. Upstream customers transmit back to the base station using Time Division Multiple Access. The standard defines two choices: either the base station and customer transceiver use the same frequency, or they operate at different frequencies. Operating the equipment at different frequencies enables synchronous transmission in both directions.
Compared with line-sight-operation, 802.16 non-line-of-sight operations must be able to cope with harder technical problems at the physical layer, such as multipath propagation of radio signals as they bounce off buildings and other large objects, which can cause problems similar to the effect of acoustical echoes.
Unlike fiber or copper cable technologies, 802.16 deployments must deal with changeable environmental factors. Rain can interfere with reception. The 802.16 specification includes radio link control to establish initial parameters when links come up as well as to alter them as conditions change. Just as cell phones adjust their power consumption in relation to their proximity to a base station, 802.16 equipment will continue to monitor link quality after initialization and will adjust transmission parameters accordingly.