Principle of the cellular communication system :
PLMNs operating on a national level are divided by location into servicing areas , so-called cells , in which a Base Transceiver Station BTS supplies the mobile subscribers of the area concerned . The cells represent the smallest service area in the PLMN network .
Roaming :
A further innovation of the cellular system was so called Roaming. This means that a subscriber can move freely within the PLMN and remain reachable on a single personal telephone number anywhere in this area . With GSM this concept of roaming can be expanded to the international area ( international roaming ). A subscriber whose home PLMN has a roaming agreement with othe countries ‘GSM-PLMNs can also be reached in these PLMNs ( Visited PLMN –VPLMN ) without dialing the corresponding VPLMNs code :calls can be also be made from that VPLMN .
Location Update :
The subscriber has to be located in the respective cellular network. A procedure known as Location Registration or Location Update Procedure (LUP ) carries out this function . It is important that the subscriber‘s temporary location area is recorded 1 registered with this procedure when the subscriber‘s mobile station is switched on and checked in , to forward calls to him . The temporary Location Area (LA ) is the area in which the M can move freely without having to carry out A location update . As a rule , the location area consists of a multiple cells and is configured by the operator according to the traffic or population density .
Handover :
In cellular networks , it is not necessary for the subscriber to have his call interrupted when changing from one cell‘s service area to the area of a surrounding cell , as long as the cell areas overlap . This overlapping should be guaranteed with good planning.
If the MS can receive better supply from another cell than the one currently in use during a call , the MS connection will be diverted to the relevant cell . This procedure designed for system quality maintenance ideally takes place without the user being able to notice and is known as Handover .
Authentication :
For authentication and ciphering , the Authentication Center (AC ) and the SIM card are important ; they store the following data :
- IMSI (International Mobile Subscriber Identity )
- Ki ( Individual Key )
- A3 ,A8 : Algorithms for the creation of authentication and ciphering parameters .
The triples are parameters , which are necessary for authentication and ciphering . they are produced in the Authentication Center ( AC ) and consists of :
- RAND ( RAND number )
- SRES ( Signed RESponse )
- Kc ( Cipher Key ) : key necessary for ciphering .
Duplex Transmission and Multiplex Procedure :
GSM uses FDD ( Frequency Division Duplex ) to separate UL and DL . A combination of FDMA and TDMA is used for GSM . The GSM physical channels are defined by a pair of frequency bands ( for UL and DL ) and a time slot Ts .
FDMA in GSM:
In the GSM system, a band width of 200 KHz is defined for one frequency band. These HF channels widths are perfectly suited to the demands for speech transmission. Allocation to GSM900 and GSM1800 is as follows:
· GSM900: (880) 890 – 915 MHz ; 925 (935) – 960 MHz; 124 (174) channel pairs ; with a duplex distance of 45 MHz
· GSM1800: 1710 – 1785 MHz ; 1805 – 1880 MHz; 374 channel pairs ; with a duplex distance of 95 MHz
In GSM for DL the higher and for UL the lower frequency range is used in general.
TDMA in GSM:
Each of the 200 KHz frequency bands is further sub-divided by TDMA into 8 so called Time Slots TS.This produces 8 physical channels within one frequency band. In GSM a physical channels is thus defined by a determined frequency channel Uplink UL and Downlink DL and a determined time slot TS In the GSM system, up to 8 (with half-rate transmission even 16)calls can be transmitted ’’simultaneously’’ on one frequency band.
A sequence of 8 time slots TS in one radio channel is referred to as a TDMA frame. A TDMA frame has a duration of 4.615ms, an individual time slot a duration of approx. 0.577 ms.
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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