The target of mobile communication system is to gradually realize personal communication using the always existent radio channel as transmission media.
However, the radio channel has poor transmission features.
Firstly, there is serious and complicated fading, including path fading, shadow fading, and multi path fading.
Secondly, the radio transmission path may be direct or obstructed by mountains or buildings. It is difficult to analyze the unknown and unpredictable elements in radio channels.
Even the relative moving speed may greatly affect the fading of signal level.
Although the features of electromagnetic waves change a lot during transmission, the major changes fall into perpendicular incidence, reflection, diffraction (inflection), and scattering.
In cities, there is no direct path between transmitters and receivers. The high buildings and large mansions cause serious diffraction loss.
Reflected by objects by many times, the electromagnetic waves reach the receiver through different paths.
The interaction of these electromagnetic waves cause multi path fading at specific place.
In a word, the strength of electromagnetic waves decreases with the extension of the distance between the transmitter and receiver.
- 1. Low Antenna of MS
Because the transmission path is always affected by topography and man-made environment, and the MS moves in various topographical environment and buildings, it makes the signal received by the MS become the increment of a large number of scattered and reflected signals.
- 2. Mobility of MS
The MS is always moving. Even the MS is not moving, the surroundings always change, for example, people and vehicles move, and wind blows leaves. The mobility makes the transmission path between the base station and MS always change. In addition, the moving direction and speed of the MS will cause the change of signal level.
- 3. Random Change of Signal Level
Varying with the time and locations, the signal level can be described by the probability distribution in random process only.
- 4. Wave Guide Effect in Metropolitan Environment
The wave guide effect caused by the high buildings on both sides of the street make the signals received in the direction parallel to the street enhanced and the signals received in the vertical direction weakened. There is about 10 dB difference between the two signals. This effect is attenuated 8 km away from the base stations.
- 5. Loud Man-Made Noise
The man-made noise includes noise of vehicles and electric power lines, as well as industrial noise.
- 6. Strong interference
The common interferences include co-channel interference, adjacent-channel interference, intermodulation interference, and near-far interference.
As the MS moves further from the base station, the signal received becomes weaker and weaker. The reason is that path loss occurs to the signal. The factors causing the path loss include carrier frequency, transmission speed, and the topography and physiognomy where the signal is transmitted.
Shadow effect: The semi-dead zone in the coverage area caused by the obstruct of high buildings and other objects.
Near-far effect: Because the mobile subscribers move at free will, the distance between the subscriber and the base station changes. If the MSs have the same transmit power, the signal strength at the base station is different. If the MS is nearer to the base station, the signal received by the base station is stronger. The non-linearity of the communication system will be worsened, making the stronger signal stronger, the weaker signal weaker, and the stronger signal suppress the weaker signal.
Doppler effect: The shift in frequency which results from the move of the signal received at high rate. The degree of shift is in direct ratio with the velocity of the mobile subscriber.