Transmission Technology

Transmission Technology:

The performance of wireless communication systems has some limitations because of the mobile radio channel. The transmission path between the transmitter and receiver can vary from a simple Line of Sight (LOS) to one that is severely obstructed by mountains, buildings and so many obstacles.

The performance of wired channels can be predicted. However, the radio channels are very random. They cannot be easily analyzed. Also as the mobile equipment moves the signal level fades. Hence, modeling a radio channel is a difficult part of mobile radio system design. A radio channel is modeled based on measurements done for a specific communication system or spectrum allocation.

The basic propagation methods are like RFID communication systems are Reflection, Diffraction, and Scattering.

Transmission Technology

The majority of the cellular radio systems operate in urban areas. There is no direct Line of Sight (LoS) between the transmitter and the receiver. There is severe diffraction loss because of the tall buildings in such areas.

As there is no direct Line of Sight (LoS), multiple reflections from different objects causes the radio waves to travel along different paths of varying lengths resulting in fading as shown in the below image. The interaction between these waves causes multipath propagation at a particular location. As the distance between the transmitter and receiver increases the strengths of the waves decreases.

Transmission Technology

For modeling the radio channel, the propagation models are based on predicting the average received signal strength at a specific distance from the transmitter and the varying signal strength at a specific location.

The propagation models that predict the mean signal strength for a transmitter-receiver (Tx - Rx) separation distance are useful in computing the radio coverage of a transmitter. They are called large-scale propagation models. They characterize signal strength over large Tx - Rx separation distances.

The propagation models that predict the rapid fluctuations of the received signal strength over very short time durations are called small-scale propagation models or fading models.

As the mobile travels over small distances, the instantaneous received signal strength will vary rapidly resulting in small-scale fading. This is because the received signal is the summation of signals from different directions. The received signal power will vary by three or four orders of magnitude (30 or 40 dB) as the receiver is moved by a fraction of wavelength.

As the mobile travels over long distances i.e. away from the transmitter and receiver, the average received signal strength will decrease. It is predicted with the help of large-scale propagation models. The average received power is found by averaging the signal measurements. Generally for cellular and PCS frequencies in 1 GHz - 2 GHz band, the average power received is the measurement in the circle of 1 m - 10 m

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