Non-line-of-sight propagation
The term non- line of sight, in English: Non -line -of-sight, short NLOS is used in communications for a radio transmission without line of sight between transmitter and receiver.
In the non- line of sight thus is an obstacle in the first Fresnel zone. In a communication path outdoors may be the obstacle, for example a house, forest or a mountain. With most wireless connections are NLOS connections.
A well-known example of a non- line of sight is the connection between the phone and a mobile radio base station ( Base Transceiver Station). Examples of wireless transmission standards within closed rooms are Wi-Fi and DECT (for the connection between the handset and base station for cordless phones ).
One obstacle in the first Fresnel zone of a radio link causes an attenuation of the signal, resulting in a shortening of the range, or an increase of transmission errors (BER ) with itself. To counter this there are the following options:
- Increase transmission power
- Use antennas with a higher antenna gain
- Increase the input sensitivity of the receiver
- Interposition of a repeater or a repeater to increase the signal strength again. The use of a repeater or relay station can also be used to suppress the NLOS connection by being built, for example, on the mountain peak. There is then a direct line of sight (LOS) from the transmitter to the relay station and from the relay station to the receiver.
A variation of the transmission frequency would also have an impact on the damping of the obstacle, but it is convenient due to the defined and controlled by the regulatory authority frequencies is not possible. Furthermore, also causes a frequency change to another free space attenuation.
Regardless of the obstacle are in communicating the laws of wave propagation and free space loss. Furthermore, it is expected to multipath ( multipath ) and interference. All these factors must be considered when developing a communications strategy, or the installation of a funct transmission path.