Radio Coverage Area
Radio coverage occurs when a geographic area receives a radio signal above a specified minimum level. WiMAX can operate up to 50 km under line of sight (LOS) and up to 8 km under non-line of sight (NLOS) conditions. Practical cell sizes are limited to approximately 5 miles.
WiMAX radio coverage varies based on the options installed and used (such as diversity transmission) in the equipment and the modulation (such as QAM –vs- QPSK), frequency and the parameters that are set.
For the most part, there is a tradeoff between data transmission rate and distance. As the modulation type becomes more efficient (more bits per Hertz), the higher the channel quality has to be at the receiver which means the maximum distance that can be used from the transmitter is reduced.
Radio signal attenuation varies from approximately 20 dB per decade in free space to between 40 to 60 dB per decade when signals travel through objects (resulting in building penetration loss). As the distance increases by a factor of ten in freespace, the signal level drops by a factor of 1000, whereas when radio signals travel through objects (walls and floors), the signal may decrease by a factor of 100,000 or more.
Figure 1 illustrates the maximum distance and data transmission rates for fixed and mobile WiMAX communication in a geographic setting. A 20 MHz wide WiMAX radio channel can provide approximately 75 Mbps of data transfer (when it is close to the base station) while the data transmission rate decreases as the distance from the base station increases.
Frequency Bands
Frequency bands are the range of frequencies that are used or allocated for radio services. There are two primary frequency bands defined for WiMAX systems; 10 to 66 GHz (the original frequency band) and 2 to 11 GHz. The WiMAX system is designed to allow operation on licensed or unlicensed radio channels.
A licensed frequency band is a range of frequencies that requires authorization for use (a license) from a regulatory agency or owner of the frequency band in a geographic area for permission to transmit radio signals in that area. Unlicensed frequency bands are a range of frequencies that can be used by any product or person provided the transmission conforms to transmission characteristics defined by the appropriate regulatory agency.
