Wireless Networks: Radio Frequency (RF)

Radio Frequency (RF)
The radio frequency spectrum is divided into frequency bands that are authorized for use in specific geographic regions. Globally, the International Telecommunications Union (ITU) specifies the typical use for radio frequency bands. Within each country, government agencies create and enforce the rules for which specific types of systems and services are used in specific frequency bands and which companies will be able (will be licensed) to own and operate these systems.

The national government is responsible for dividing the available frequency bands for licensing to users and regulates what the frequencies may be used for. The legal right-to-use of this public resource is controlled by rules and licensing of very specific frequencies, a range of frequencies or a block of sub-divided channels at a given frequency or frequency range.

For example, the frequencies allocated for FM radio must be used for the purpose licensed; that is a combination of music or news and public information. FM radio stations are not licensed to broadcast a secret “Morse-code” to a following of undercover militia! Neither can a “Paging Service” use one or all of their frequency channels to broadcast radio. However, with the recent deregulation of telecommunications services, wireless service providers are now permitted to offer many new types of services provided they can fulfill their basic licensing requirements.

To prevent unwanted interference from radio devices, the reckless use of transmitting energy or information on our public airwaves according to publicly published rules or licenses will violate federal law. Such transmissions are subject to prosecution or suspension of the radio operator’s license.

Frequency Allocation Charting
There are thousands of wireless applications that are assigned to many different frequency bands. The selection of the assigned frequency bands is determined by a variety of factors including the radio propagation characteristics and the availability of radio channel frequencies at the time.

Because most of the frequencies have already been assigned to licensees, a new assignment of frequencies typically requires existing licensees or users to stop using a band. These users are typically shifted to another band. This process is called re-allocation.

Historically, major re-allocations are done in the higher frequencies to avoid congestion. This has advantages and disadvantages. The radio frequency (RF) devices employed within the newer systems are subject to more loss based on distance. This requires closer distances, increasing the total number radio sites to cover the same area previously covered by radio devices at a lower frequency. However, the higher frequencies tend to penetrate buildings more readily and the antennas involved are physically smaller - both important attributes for systems that seek to reach 100% of the available population.

RF Channels and Bandwidth
An RF channel is a communication link that use radio signals to transfer information between two (or more) points. To transfer this information, a radio wave (typically called a radio carrier) is modulated (modified) within an authorized frequency band to carry the information. The modulation of the radio wave forces the radio frequency to shift above and below the reference (center) frequency. Typically, the more the modification of frequency, the more information can be carried on the radio wave. This results in RF channels typically defined by their frequency and bandwidth allocation.

Bandwidth allocation is the frequency width of a radio channel in Hertz (high and low limits) that can be modulated to transfer information. The amount type of information being sent determines the amount of bandwidth used and the method of modulation used to impose the information on the radio signal.

A government regulation agency (the FCC in the United States) defines a total frequency range (upper and lower frequency limits) that a radio service provider can use to transmit information. In some systems (such as AM or FM radio station broadcasting), this is a single radio channel. For other systems (such as cellular, PCS, or PCN), this is a range of frequencies that can be sub divided into smaller radio channels as determined by the radio carrier. When the allocated frequency range is further subdivided into smaller allowable bands, these subdivided areas are referred to as channels.

Mobility and Fixed Wireless
Most applications use wireless to allow mobile service. However, many fixed applications of wireless are practical. There is a general data transmission rate tradeoff between mobile and fixed wireless systems. Mobile wireless systems have a relatively low data transfer rate (typically below 28 kbps) while fixed wireless systems can have data transfer rates that exceed 45 Mbps. The primary advantage of fixed wireless service is the ability to focus radio transmissions to a particular direction or region. This typically reduces interference to and from other radios and increases the capacity (data transfer rate) available to the fixed wireless device. The basic types of fixed wireless systems in use include wireless computer networks, competing wireless television systems, and wireless local telephone service.

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