Transmission Problems and Solutions in GSM Network
•Signal degradation can be classified by type :
1.Path Loss
•Due to distance covered by the radio signal, “Free space path loss “LFS = 32.44 + 20 log F (MHz) +20 log d (Km)
2.Signal attenuation
•Resulting
from shadowing effects introduced by the obstacles between transmitter and
receiver
3.Fading of the signal
•Caused
by numerous effects all of which are related to the Radio propagation
phenomenon
•The Radio propagation Phenomenon :
1- Reflection :
- propagation waves impinges on an object which is large compared to wave length.
2-Diffraction :
- Radio path between transmitter and receiver obstructed by surface with sharp irregular edge.
- Waves bend around the obstacles , even when LOS doesn't exist .
3-Scattering
- object Smaller Than the wave Length of the propagation waves
These phenomena's of the radio propagation causes multi kind of fading:
1.The normal fading.
2.The multi path fading.
Note:-
Fading means that The signal strength received fluctuates around a mean value while changing the mobile position resulting in undesirable beats in the speech signal.
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Fading Problem
1.Shadowing (Normal fading):
•The
reason for shadowing is the presence of obstacles like large hills or buildings
in the path between the site and the mobile.
•Multi
path scattering from nearby objects.
•Attenuation
effects
2.Rayleigh Fading (Multi-path Fading):
•Due
to the reflected signals from the surrounding building.
•So,
a combination of direct and out-of-phase reflected waves at the receiver yields
attenuated signals (i.e. signal paths of different paths with different times
of arrival at the receiver)
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Fading Solutions
1-increase Fading Margin :
2-Antenna Diversity (Space Diversity):
- the Cell transceiver will use two receiving antennas instead of one . they will separated by a distance about (10 * λ ) and they will receive radio signals independently , So they will be affect differently by the fading dips , and the better signal received then will be selected .
3-Frequency hopping(Frequency diversity) :
-Frequency hopping mean each time the BTS or Mobile transmits a burst on a different RF Carrier Frequency.
-The Hopping must use a predetermined Sequence , So that both transmitter and receiver must hop in synchronization , this sequence is stored in frequency hopping table .
Time Delay Problem
- Transmission Delay is unavoidable in the radio interface , if the mobile station moves away from the base station during a call , the further distance the more delay , the Uplink is set same.
-If the delay is too high , the time slots of the signal from a certain mobile station and that of the next signal from another mobile station received by the base station will overlap each other .
Time Delay Problem Solution
-To
avoid this, during a call, the measurement report sent from the mobile station
to the base station carries a delay value. Moreover, the base station should
monitor the time when the call arrives and send an instruction to the mobile
station via the down-link channel every 480 msso as to inform the mobile station
the time of advance transmission.
-This
time is the TA (timing advance), which ranges between 0~63 (0~233μs ). The TA
value is limited by the timing advance code 0~63bit of the GSM system.
Therefore, the maximum coverage distance of the GSM is 35km. Its calculation is
as follows:
-1/2 x
(3.7 μs/bit) x 63bit x c = 35km
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