Saturday, October 3, 2009

Radio Propagation | WiMAX Radio

Radio propagation is the process of transferring a radio signal ( electromagnetic signal) from one point to another point. Radio propagation may involve a direct wave (space wave) or a wave that travels along the surface (a surface wave). Radio propagation characteristics typically vary based on the medium of transmission (e.g. Air) and the frequency of radio transmission. WiMAX systems may be operated as line of sight (direct transmission path) or non line of sight (indirect transmission path) systems.

WiMAX systems are typically designed with a radio link budget. A link budget is the maximum amount of signal losses that may occur between a transmitter and receiver to achieve an adequate signal quality level. The link budget typically includes cable losses, antenna conversion efficiency, propagation path loss, and fade margin.

Due to transmission impairments, a fade margin is budgeted in a communication link. Fade margin is the amount of signal loss, usually expressed in decibels, that a radio signal in a communication path is anticipated to change (or budgeted to change). This helps to ensure that typical signal fading periods do not result in a lower than expected quality of service.

One of the key types of signal fades that occur on microwave systems is a rain fade. A rain fade is the signal loss that results from signal absorption and scattering in water droplets (rain).

Line of Sight (LOS)

Line of sight (LOS) is a direct path in a wireless communication system that does not have any significant obstructions. WiMAX systems that operate in the 10-66 GHz range are LOS systems.

Non Line of Sight (NLOS)

Non line of sight (NLOS) is a wireless communication system that does not have a direct path (can have significant obstructions) between the transmitter and receiver. NLOS systems can use radio signals for transmission.

Figure 1 shows how non line of sight (NLOS) radio propagation can allow a radio signal to reach its destination in congested areas. A radio tower is transmitting through an urban area which does not allow a radio signal to travel a direct path from the tower to the receiver. Accordingly, multiple alternate paths are reflected off a building to reach its destination. A main signal (shortest reflected signal) and another signal (delayed signal) become part of the received signal.

Figure 1: Near Line of Sight Radio Propagation
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