Radio Interference:
The air or radio interface, i.e. the connection between the MS and fixed network components, represent the fundamental difference to a fixed network telecommunication system. The radio interface has its specific advantages, but also shows problems and disadvantages inherent to mobile communications.
The main advantage of mobile communication is the unrestricted mobility which can be achieved only via a radio interface. Mobility was extremely restricted, especially in the early years of mobile communications (one-cell systems).
There are many problems of radio transmission like fading, path loss time dispersion, time delay, interference, etc.
Fading:
Fading means that the signal strength received fluctuates around a mean value while changing the mobile position.
Slow Fading = Log Normal Fading = Shadowing:
The reason for shadowing is the presence of obstacles like large buildings or hills in the path between the site and the mobile. The distance between fading dips (minimum values of signals strength) is from 10 to 20 meters.
· Solution For The Slow Fading:
To overcome the fading problems, the fading margin should be large enough that the lowest fading dip is still higher than the receiver sensitively.
Fast Fading = Rayleigh Fading = Multipath Fading:
This occurs when a signal takes more than one path between the MS and BTS antennas. The signal is reflected off buildings, for example, and is received from several different indirect paths.
Rayleigh fading occurs when the obstacles are close to the receiving antenna. The received signal is the sum of many identical signals that differ only in phase (and to some extent amplitude).
· Solution Of Rayleigh Fading:
1. Antenna (space) Diversity:
The cell transceiver will use two receiving antennas instead of one. A distance of about 5 meters will separate them, and they will receive radio signals independently, so they will be affected differently by the fading dips and the better signal received will be selected.
2. Frequency Hopping:
The fading effect may not be the same for all of the frequencies, so the time slot of the subscriber will jump or ''hope'' between the frequencies of the all cell when it is repeated in each TDMA frame. If only one of the frequencies is affected severely by fading, a small fraction of the signal will be lost. The hopping sequence might by fading, a small fraction the signal will be lost. The hopping sequence might be cyclic or random.
Pathloss:
Pathloss is the reduction in power destiny (attenuation) of the signal as it propagates through space.
Loss=32.4+20 Log f (MHz) +20 Log d(Km)
· Solution of pathloss:
Increase the transmitted power but by limits to compromise the frequency reuse.
Time Delay:
Each MS on a call is allocated a time slot on a TDMA frame. This is an amount of time during which the MS transmits information to the BTS. The information must also arrive at the BTS within that time slot. The time alignment problem occurs when part of the information transmitted by an MS dose not arrive within the allocated time slot. Instead, that part may arrive during the next time slot, and may interfere with MS and the BTS causes time alignment. Effectively, the signal cannot travel over the large distance within the time.
· Solution of Time Delay:
Timing advance is a solution specifically designed to counteract the problem of time alignment. It works by instructing the misaligned MS to transmit its burst earlier or later than it normally would. In GSM, the timing advance information relates to bit times.
Time Dispersion:
Time dispersion is another problem relating to multiple paths to the Rx antenna of either an MS or BTS. Time dispersion causes Inter – Symbol Interference (ISI) where consecutive symbols (bits) interfere with each other making it difficult for the receiver to determine which symbol is the correct one.
This problem does not occur if the Distance between the MS and The Building as example that cause the reflection not increase than 0.5 km as distance related to one bit processing.
· Solution of Time Dispersion:
1. Increase Carrier to reflection Ratio (CIR).
2. Viterbi Equalizer:
This is an equalizer used to equalize the effect of at most four bits delay. A bit pattern called the ''Training Sequence'' that is known to the mobile is transmitted with every burst. The equalizer compares the received pattern with the expected one and creates a mathematical model of the channel that probably caused the difference between the two patterns. Then a probable transmitted bit sequence is fed to the channel model and the output is compared with the received bits until reaching the most probable bit sequence.
Interference:
· Co-channel Interference:
It is the interference caused by reusing the frequencies in near distances (decreasing reuse distance).
· Solution of Co-channel Interference:
1. The GSM specification recommends that the carrier -to- interference (CII) ratio is greater than 9 decibels (dB).
2. Sectorization of Cells, Then n can be reduced to be equal 1.
· Adjacent Channel Interference:
Is the interference between Carrier frequency and adjacent frequency. Adjacent frequencies (A), that are frequencies shifted 200 KHz from the carrier frequency (C), must be avoided in the same cell and preferably in neighboring cells also so.
· Solution of Adjacent Channel Interference:
1. The GSM specification states that the carrier-to- adjacent ratio (CIA) must be larger than -9dB.
2. Also Sectorization of Cells as in co-channel interference.
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